I've always pretty much figured the tanks are there because you couldn't hook up where you were camping, and thus must travel with the stuff on board. Most of our camping is in remote forest service/blm etc campgrounds where at best you'll have a pit toilet in the campground, and maybe a water spigot. Dump stations are rare.
To be honest, even in trailers I've had with underpowered trucks, I never noticed any significant MPG or handling differences between traveling tanks full, or tanks empty. My current TT has 2 40 gallon grey tanks and 40 for the black and even when they are fairly full I don't seem to notice any difference even on long mountain dirt roads.
