Originally Posted by muddy_hollow
Anyone done this type of thing?
Yes. The Airstream Factory, from 1969 to present.
Seriously, I'm sure this design would be quite adaptable to vintage units. I think this is in "Safari Tim's" plan for his ambassador.
70's units had a custom tank made to take up 2 frame cavities, so there's a generous 45gallon capacity. I'm sure you could accomplish the same thing using 2 seperate tanks...or if ~20g is enough, just one. You can also rig it w/ the plumbing so that it fills from the city connection, although, if that's the only method, you'd lose the ability to fill it w/ a bucket. (and I've done that, in a pinch).
Put the drain along the back edge, then the tongue jack will give you all the slope you need.
the 70's tanks were (are) flat. and it does take a long time to drain on mine, anyway...the drain is only a 3/8ths pipe. it can take a couple of hours to empty it.
Since you're going to be doing frame mods, I'd say you could probably even get one of these specific tanks...but they're meant for a 5" cavity.