I have an 11 gallon one. I know it is small, we had it from the PU days and never had a need to get a bigger one yet.
I do daily choirs and taking out 11 gallons of gray water every 2 days is enough. Sometimes 11 gallons goes more than 2 days. I have to fill the fresh if we use a lot more. We can easily go 4 days on the gray tank until full, with showers and dishes. So taking 11 gallons out every 2 day works for us. I do not have to drain it dry, just keep ahead of the daily use.
Since our longest stay in one spot is 8 days, this is not a problem. Pending how long it is to the dump, I strap the handle to the tow ball and pull it or I can still lift it into the bed full. Much more than 11 gallons and I do not want to fiddle with it lifting it.
I fixed up a shorter sewer hose and a bayonet fitting so it dumps from the pipe into the tank and I have to vent the tank. There really is no mess if you are careful. Do not pull the handle wide open, I only bump it and see what is coming as mine having a longer dump pipe from the tank does not shut off real quick. I push the valve close and it keeps coming for a bit.
I also have done the black tank, again only taking 11 gallons out to hold up over several more days until we break camp. Same process as the gray, just go slow filling.
Mine is this one 11 gallon Tote-n-stor
Amazon.com: Tote-N-Stor 25606 Portable Waste Transport - 11 Gallon Capacity: Automotive
John