I have this 12 gallon model from Thetford…
Thetford 40505
https://a.co/d/gOrmiG5
It is just enough to dump the SeaLand toilet. I can stow it on the spare tire rack underneath the camper.
HOWEVER- It comes with no accessories whatsoever, including a tow strap to drag it behind your tow vehicle.
https://a.co/d/60p2pVP
By the time you buy a small sewer hose and adapters etc you probably could get a bigger toter but then you run into storage issues and height issues. The dump plumbing on a TM is pretty low to the ground and the inlet attachment on a lot of the toters out there are taller than the dump plumbing. Of course the other reason I bought this one is because I got it for super cheap new in box on CL.