Lookin good!!
I would suggest fabing a side door, allowing for access in front of the tank. I would also suggest that the water tank be mounted 2/3 forward of the axle. This compensates for the weight of the generator, pressure washer, reels, hose, compressor, etc. when the tank is below 1/2 -- it'll also help in being able to move it around when it's not hooked up to a trailer even with a full tank.

PM me and I'll send ya some pic's of my 4x6 enclosed showing some of the little custom things that make my job easier -- like exhaust through the floor, electric brakes, pressure washer vents and custom 18x24 side door.
If ya don't mind, how much are you into this trailer ($$$)?