Unfortunately, that 5 or six gallon estimate is pretty close to accurate, as these things are plumbed on the side of the tank, on a flat tank at that, and need to produce suction so that the water must stay above the exit hole. Being level when using the tank is about all you can do to maximize the most out of the tank, unless you can verify where the exit location is, and tilt the camper more towards that direction to get more out.
I carry 3 extra 7 gal. water jugs for that reason; mine has a 30 gal. h20 tank, I think. Randy
/SIGPIC]'08 V-lite Flagstaff 30WRLS
'06 Ram 1500 QC hemi
Reese dual cam sway control,
K&N series 77 intake, Hellwig helper spgs. LT tires,
Flowmaster "true duals", 380 h.p., Bilstein shocks