I have the 35 gallon Thetford Smart-tote. Both of my tanks are 32 gallons and I didn't want a portable tank that was going to run over. Yes it is heavy,and depending on the landscape it can be hard to pull, but with the two of us we manage to get it to the hitch and I have no problems after that. I also like NOT having to tilt it to empty it.
Like CincyGus, I have on occasion had to empty the Black tank. But I always follow it with the gray water. The 35 Gallon tank also has the built in "quick rinse" sprayer with hose attachment.