As each layout and conditions vary, there is not pat answer, but the
general idea is in order to keep the bottom of the trailer streamlined
and critterproof - and to allow room for the dump valve, in a retro-fit
solution you end up using about an 18" x 22" wide or so by 10"-11" high
tank, on which the toilet sits on top, sandwiching the bathroom floor to
the tank, supported from the bottom by straps running between two
frames. Alternatively, you can have the tank fabricator attach an L
flange around the mid section of the tank and support it from the floor.
A black water tank has to relatively square and not too wide. A 5" high
but long tank works for gray water, but would not work for a black water
tank for obvious reasons.
Tank suppliers are all over the place and ship. They will all build to
spec, including the toilet, dump valve, vent pipe and any rinse head or
sensor flanges you desire. It is a good idea to have all these
attachments in hand before ordering a tank, so you can spec out the
correct size threads and the like.
Couple of companies are listed here:
http://vintageairstream.com/rr_topics_suppliers.html#parts
Best of luck,
RJ