Subjectively, a 7x16 box is a SMALL starting point for everything you're planning to do.
Kudos to you for being creative and not buying a RV pre made. Everyone has their own ideas of what is "right" for them.
While a TT would be easier and more practical, it's your money. But Id consider enough trailer to house ALL the living amenities you'll be adding and not over complicate it by having to link the truck and trailer together.
Also collapsible water storage is preferable IMO especially in your anticipated off the grid scenario. For your 50 gal transfer tank and pump setup you're planning, you could get 100gal or more of bladders or fat sacks for wake boats and a ballast pump and when you don't need the water, the "tanks" each fold up like a sweatshirt for storage.
