"These trailers are flat on the bottom "
Most I've seen have a frame underneath.
You won't be easily able to 'hide' solar panels on top.
For the kind of conv we're talking abt the water tank can be a W-M 6 gallon 'green' tank near the back on each side with a small pump routing the water to your sink via the B-R with an accumulator tank in there with a couple valves and a pressure guage,
Tanks secured with a cut down ATV strap each end hooked into the E-track you've optimally mounted.
Our 'green' W-M tanks have about 70,000 bouncy miles on them and are going strong. Works great!
We put a Fantastic vent in the B-R and an unpowered one in the back over my head.
Works great for a cross flow of air until it rains. If I'm sleeping and want it dark I stick a small pillow into the vent and pull one of the 5 [each side] ATV type bedding hold UP straps across tight under it to hold it in there.