In the early 2000's there was a recall on certain 21 ft Bigfoot models because of the poor handling qualities of them. The recall added 500 lbs of steel weight to the tongue. There are a number of them out there that didn't get modified and there is a story about a couple that bought one, not realizing the issues with it, and totaled the trailer and their tow vehicle on the way home with it.
Whether you want to or not, you could end up having to add a couple of sections of Rail Road track underneath the tongue area.
As far as water goes, get some blue 5 gal potable water jugs and carry the water in the bed of the truck and fill the tanks after you get to the site. This is not a great option but should help some.
Where is the spare tire? Certainly it can be moved to a holder underneath the front of the trailer.
I would consider moving the water tank. It is probably just braced up or strapped up to framework underneath. Move it forward install a new filler neck and tube down to it (might have to build a box around the fill and vent tubes) and remove the existing fill neck and install a blanking plate over the opening where the fill port was.
I have a Thor trailer with a couch across the front that does the funny flip routine to make a bed. The back swings away from the wall and rotates at the same time till it ends up in front of the seat cushion part of it. The seat cushion frame lifts up to expose the pass thru. You may be able to remove a piece of plywood and get into the pass thru if the seat bottom lifts up. Mine pivots up toward the front wall once the back is out of the way.
Charles