i've been using an ~18g rubbermaid bin as a resevoir for my RO/DI water for the longest time. Some people even use them as sumps. Saw on RC one person's shark tank consisted of about 6 "outdoor" garbage cans hooked together in series with uniseals. 300 gallon sump for aproximately $150
I'm going to switch out my rectangle shaped bin for a round garbage bin pretty soon though, cuz the sides bulge out when the thing's almost full.
I like the idea of putting it on a cart though
I'd like to put my resevoir on an elevated platform, and just use a hose through a uniseal. I'll just make the hose long enough to reach my tank and put a valve at the end so I can turn it off without going back to the laundrey room. Might as well make the platform a cart, just in case I would like to wheel it right in front of the tank for some reason. Never know.