I'm installing one meant for tent trailers between the frame rails on my boler
Three are shown stacked on top of each other taken from post # 9 here:
Grey water tote
That link provides a link to this PDF
, which gives some sketches on page 9 of the frame that holds it.
The frame is hinged at the rear and drops down at the front (which is actually the rear of the trailer) so the tank can be stored underneath empty and it rolls out when needed. They were and are used in a number of different tent trailers
I found mine on ebay, but don't see any today. Link to one for $115 without the frame: Barker Folding Tent Camper Tote-Along, 15 gallon - $115.18
You could buy the tank and fabricate your own frame from the diagrams.
You could probably pick one up at a RV Salvage yard. This list is updated regularily and should be added to your favourites:
RV Salvage/Surplus Yards (List) Updated 9/3/13 - RV Forums - myRVparks.com