We have a shower tent that we have used at the Fort. To prevent the water from running onto the ground, you have to get a decent size rubbermaid container to stand in for a "tub". You can always dump that water into a bucket to make it easier to carry to a comfort station. If you're using water from a solar shower bag, you won't use a large amount of water, therefore carrying it to the comfort station wouldn't be an issue.
To be honest, though, the comfort stations are really nice. I'm a complete germ freak, but I finally gave the comfort station a try this last time. I'm SO glad I did. They were clean, and the water was reliable - which is much more than I can say for our shower tent. It was either too hot or too cold, and at one point a baby lizard must have been at the water spout when DH filled the bag. The lizard ended up IN the shower bag. Needless to say, I didn't use that shower bag. lol