We ended up going with two smaller 15 gallon barker tanks instead of one big one. The weight of one full 15 gallon is 135 lbs, which I can get into the truck bed. Any heavier and it would have to be towed. We usually are pretty far from the dump station at the places where we camp, so I didn't want to limit the drive speed to 4 mph.
We started with one, but having one only allowed me to empty only half of the gray tank. DW and DD both have long hair, so their showers can use a lot of water. After having to setup/dump/load/drive/dump several times in one weekend, I realized that the second tote would have saved me about 20 - 30 minutes of setup and drive time over that weekend. That's a good enough percentage of a camping weekend to justify the second tote.