Dunediver,
Most people here, including the lurkers who never post, get a great deal of helpful insights and workarounds by cruising the various posts and photos. I know I do, and many have said so straight out. We all appreciate eachother and have Mike and his helpers to thank for creating and maintaining this wonderful network!
Confession: I've built motorhomes from scratch with ALL the bells and whistles (full plumbing and electrical, banks of photovoltaic panels and wild raised roofs with sleeping lofts. I joined a 12-step program for DIY addicts. It didn't work, but at least I downsized lately!
Here's the deal: You want a toy hauler done your way, not too big or small and concise. I get that. I would advise keeping it to the bare necessities. If you really have to have running water, an RV 12 volt DC pump (Shurflo brand) will power all the faucets. The problem with a cargo trailer is putting the gray water and poo tanks underneath. There's not much ground clearance. So if you get really shallow tanks, there won't be much volume. Frequent runs to the dump station are a pain. I prefer frequent runs to the showers and outhouse instead of installing all that and hauling waste that around. As for solar panels, get the biggest one you can find, put a voltage regulator on it and wire it to two group 27 deep cycle batteries. You'll have plenty of power if the sun is shining and you're not parked under a tree. I like framed and flexible Uni-Solar panels (no breakable glass). They quit making them but you can still find them occasionally on eBay.
Go read all the posts here and you'll have even more answers to your design questions. A dozen brains are better than one (meaning mine).
Keep us posted!
Prem
My goal...
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...is to live in a trailer.