We have the 18 gallon Thetford tote tank. Haven't used it much but when you need one it really helps. At 18 gallons, it's light enough for two of us to lift into the back of the truck so you don't have to tow it slowly and noisily way down to where the dump station is.
Even at only 18 gallons, it's still big and the only place for it is in the back of the truck. Some have made something to store them under their camper. Someone had a pic of one mounted using a BAL spare tire carrier.
Some have replaced the stock plastic wheels with pneumatic ones. I'll probably do that one of these days. Barker has a model with pneumatic tires.
You can buy a pillow (bladder) tank for grey water like
here. I wish you could get one for black water then you could just roll it up when not needed.
Here is a top 10 review list, except only has 7 on the list...
Top 10 review I didn't know that some tanks only have 2 wheels. Think I'd rather have 4 wheels.