I am also trying to design a dive trailer from a 5'x8' enclosed cargo trailer. Trying to make sure I have everything I could possibly need in the design so that I do not forget anything is a PITA. My worst fear is getting it all finished and then remembering to add something and having to start all over again.
So far I will have storage brackets for 12 scuba tanks (I have 7 currently so that gives me a little room to grow). I am wanting to have a hanging rack for wetsuits, BCD's, etc., storage for 2 DPV's plus spare batteries and chargers, 2 kayaks mounted on the roof plus assorted kayak gear storage inside, storage area for my dual-tank O2 kit, first aid kit, trauma kit, AED, 2 Rescue Cans mounted to the wall, fire extinguisher, storage areas for assorted dive gear and parts, a work bench, small BBQ grill, and storage for a 10'x10' pop up canopy and some folding chairs and tables.
I have mounted 4 dome lights inside, and "work area" lights on the back and right side of the trailer for night dives.
If at all possible I also want to have room for a small bathroom for emergencies where I can have my chemical camping toilet installed. I would also like to somehow find room for a rinse bucket to rinse my gear on-site instead of waiting until I get home to do it.
I know that is a lot to try to squeeze into such a small dive trailer, thats what is so frustrating, trying to find a way to utilize every inch of it so that it all fits.