Correct. If the MPPT controller is in use, even if it's 1 watt, you lose half the alternator charging, so you want either a high amp relay so you can switch it on and off remotely, or a selector switch so you can turn it off.
I'm using a selector switch, because I have an off-road trailer with 500 watts of solar, so if I have items on the roof (blocking the vehicles solar panels) I can solar charge from the trailer, or both trailer and Rover, or no solar charging. Only issue is I need to manually select the solar input.

 

You could cut off the solar panels automatically based on alternator activity. A voltage sensing switch would do that. Could be done with a high side driver instead of a relay (more expensive but comes with built in current limit, current sensing and it doesn't wear). I also wonder if you can control the Renesys via its RS485 port somehow.

 

I was watching Ronny Dahl's new episode on dual battery systems this morning and saw that he has a very narrow 100aH lipo4 battery in his vehicle so I decided to do some digging and found a couple of options that might fit behind the rear cargo bins.

https://kickassproducts.com/collecti...0ah-battery-fg

 

That particular battery is great for going behind seats, as it is not deep, but it is long and tall, making it harder to place in the storage bin in the Disco Cargo area, if you have a fuse panel in the same bin.