Like you, our camping is mostly full hookup, but I did buy a Barker (18 gal) for about $100 for use in state and Federal parks. Use it for either black or gray tanks, no problem; doesn't handle the full quanity of either tank but 'makes room' and that's good enough. Didn't want to go any larger, because even though I pull it to the dump stationwith the truck, I still have to pick it up off the hitch and roll it to the 'drop zone' and tilt it to dump. 150 lbs full weight is enough for me. Looked at the Thetford, thought it was overpriced and not as well built as the Barker, but that's me.