I use a WDH on my 00 Disco II for short trips with a 24 foot HiLo travel trailer (hard sided but cranks down for transport, then back up again when stopped) that weighs 3800 dry (about 4800 loaded). The WDH has never given me a problem and makes an enormous difference in rear squat and driveability. Getting up hills and maintaining highway speed is another issue--the transmission and engine have to work at an alarming rate. I should add that my setup includes a sway bar (friction); I have towed both with and without the sway bar using my Rover, and the difference is VAST when a truck passes or wind is an issue. My normal tow vehicle is a Nissan Titan. Also, I knew the OP is considering surge brakes on a boat setup. IMHO surge brakes will put a Disco in way too much trouble before they are a help ( am thinking about panic stops), and electric brakes should be used for all other setups that are over 3000 pounds. My 00 D II brakes are WEAK (well maintained, properly bled, still WEAK) when towing such a load. Slow to start, slow to stop, and at the whim of strong wind gusts. Finally, as a bonus, the Disco is fairly narrow and without some special tow mirror setup, seeing around a wide trailer is impossible. Pluses are that not seeing behind me, I do not stress over the line of cars, all pissed off, behind me. Also, backing the trailer with such a short wheelbase vehicle is a snap as long as you don't want to back straight.

As to the other question, I wouldn't connect your battery to the vehicle for charging on an always on connections. Aside from draining the battery, those wires are extremely thin for a charging setup and if they melt and ground, you will be in a world of poop trying to track it down.