Natjwest, I use nothing more than a Perko marine battery combiner switch in the camper.I use the simple Perko switch from West Marine part # 281469 for mine. It has 4 positions: Battery 1, Battery 2, Both, Off. This allows me to run the camper on a combo of the truck and camper batteries, camper battery only, or truck battery only. When switch is in the “Both” mode then the truck and camper batteries are in parallel, and, of course the camper batteries help start the truck. I generally leave everything in parallel on “Both” setting. If you really want the camper battery to help start the truck, then you’ll need heavy cable from the truck batteries back to the camper.
There’s a 10 amp multi stage marine battery charger in the camper. When I’m hooked to shore power everything is getting topped off by the charger. I ordered a 100 watt solar panel (http://www.amsolar.com/rv100.html) but haven't installed it yet. I'll probably just run the camper on it's own batteries once the solar is installed.
Even more confused now ?
I run mine the exact same way. Been working perfectly for six years now. And its only 10 guage wire to the camper from the truck.