Our camper guy suggested the same thing, but could not find a way to get a steady 48V from the hybrid system. The camper has a 48V input. My feeling is keep it simple for now, but will research this and if anyone has any ideas, please let me know.

 

Regardless of the voltage involved, you really should have a DC-to-DC charger setup between the Defender's charging system and the trailer batteries. Victron is the go-to brand for these types of things. The charger protects both the vehicle and the house batteries. Likely because I'm not familiar with the 48V system - I personally would not be hooking up anything to that side of the electrical system and I would research how to get the 12V system to charge the 48V house batteries.

 

I will second that you need to have a DC to DC converter. Our Defenders have a smart alternator and put out anywhere from 12v to mid 14v depending on the vehicles BMS which is optimized for an AGM battery. Assuming your trailer's battery is lithium(lifepo4) the defenders charging profile would not be good for the trailer battery long term. Also make sure you put the DC to DC converter as close to the charging battery as possible. So I would put it in your trailer. As for DC to DC converters I will second that Victron is very good and they do make a 12v to 48v option though it only puts out 8A. Redarc makes a pretty good one now. It is neat because, if you were to do a second battery in the defender, it allows you to charge the second battery of one type of chemistry while still allowing you to charge the main battery with a second type of chemistry once the second battery is full via the solar input. It is kind of expensive but Redarc are a solid brand. Renogy is also another popular one(used in the defender for the second battery). I have had mine for two years now. I got it supper cheap from facebook marketplace for $75. Not bad for a 50a converter. There are plenty of other brands out there too that will work in all sorts of budgets.

My trailer has an enerdrive converter/charger. What is nice about this unit is you can set your charging Amperage to match what your trailer batteries and tow vehicle can handle. It allows you to set a low amperage mode when the batteries are full but your trailer is still consuming power(for fridge and other accessories). For mine I set the charging to 30A and the lower amperage mode to 6A. It also has a solar input that I use to deploy additional portable solar panels.

 

I am getting my trailer back from the dealer soon and was planning to do an Anderson plug from the defender to the trailer to charge the 400 ah battery bank that came with it. I have been YouTubing a bunch of videos about it. The trailer has a mastervolt system in it. The combimaster is a 12/3000/160 it is an all in one system supposedly it is a very good system. Mine is being replaced because it went down in the middle of our vacation last year. It’s taking eight months to replace. I am thinking of going with the red arc for my DC to DC controller and just tapping the battery under the passenger seat and running wire inside the vehicle to a Gromit. I would like your thoughts and suggestions and any pictures you might have thanks guys much appreciated.

 

You don't mention your battery chemistry. Make sure you know what you have. The mastervolt system you reference is an inverter/charger for the camper and it appears to me it is designed for an AGM type battery. This system also does not have the capability to take 12vdc from any source to charge the battery. It is expecting you to plug it into the wall and charge it up that way. The combimaster converts 120v to 12v for charging. With all that being said you will need to buy a dc to dc converter if you want to charge from your vehicle or any other 12vdc source. Redarc is a good brand and will work fine, but please make sure you verify what battery chemistry you have. The charge rate for agm or other lead based battery is significantly lower and it would be a waste of money to by a high powered DC to DC charger if you can't fully use it, unless you plan to upgrade your batteries sometime soon. Most lithium based batteries can take anywhere from 50% to 100% of there capacity as there charge rate. This means that if you have a 100AH lithium iron phosphate (LiFePo4) you could charge it with a 50 to 100 amp current based on battery specs. Lead based batteries are much less than that.

Ok with that stuff out of the way.
I would by the appropriate redarc charger, if LiFePo4 I would get this one, if lead based same model but less amperage output. Then install it near your batteries in the trailer. It is ok to have more than one charger connected to the batteries. Then run a wire from the DC to DC charger to the front of your trailer with a anderson connector on the end. PS redarc has some pretty good wiring diagrams in there manuals. The charger I linked to is typically used in dual battery setups in a single vehicle, but it will work here for what you want to do.

As for the defender, you could wire from the battery under the passenger seat to the back, make sure you get both the positive and negative wires. Also make sure you attach the negative wire upstream of the defenders BMS, otherwise the alternator and charging algorithm might not charge the defenders battery correctly. The second option is to wire from the 48vdc to 12vdc DC to DC converter under the floor plate in the back of the defender. Much shorter run that way. I prefer this option as that converter is only powered up when the engine is running. This would keep you from accidently draining the main battery if you left your trailer plugged in with the tow vehicle turned off. FYI most DC to DC converter have a input signal that doesn't allow the battery to be charged unless you have an ignition signal. This is hard to do if you put the DC to DC charger in the trailer as now you have run a third wire to send the signal. Not very practical in my opinion.

Once you decide on a source for your power I would then figure out if you want a separate power switch for the system and where to put the fuse. Remember the fuse is to protect the wire, not the equipment. So size the fuse for your wire size. The power switch is not technically needed but I would highly recommend it. If you don't have a separate power switch than the anderson plug will be live on the back of the vehicle all the time when the vehicle is running. And if you ever get rear ended and that wire is live and goes to ground and you didn't fuse it properly there is a potential to start a fire. The power switch can be something as simple as wireless relay. I have used these before. They are pretty simple and allows you to turn on the relay via your phone through bluetooth. It is nice because you don't have tap into anything else or run additional wires.
Now once all that is figured out you just have to get the wire outside the vehicle and an anderson connector mounted. The two threads I link above detail some more of this info. There are plenty of ways to do this and multiple vendors out there with equipment to accomplish this. Basically just trying to say other may have better or less expensive ways to do this. I am a big fan of not wanting to screw with the Defenders main electrical system to much and making sure I don't cause my vehicle to catch on fire because of some stupid wiring mistake I made.

 

I know the camper has a controller to receive the power from the vehicle battery to the lithium cells. What it is, I do not know, but I the dealer will be able to tell me.

 

I need to find out from the dealer what the camper has to charge the lithium batteries. It is wired to receive vehicle power as well as solar from the camper panels and additional portable ones.