We have only camped a few times in electric only sites. I use two 6 gallon jugs to refill the fresh water and a blue tote to empty the tanks.
As far as room and height for everything, I can only give you some info that I have noticed. Our fifth wheel sits about five inches above the bed rails. I try to keep everything at the height of the bed rails. You will have movement and twisting of the trailer and need this clearance due road conditions and getting in and out of different camp sites. I have a long bed and have some room in the front of the bed for taller items.
You could carry a Gen on a reinforced bumper on the trailer. Then redesign the water tank for the truck.
