How so? What harm, exactly? The soap I use, (dr. bronners) is vegetable based and literally only a drop or two is used. The water was already there. I can't see it doing any more harm than the chemicals in your poop and pee, along with no telling what else your trip impacts (plastics off-gassing, car exhaust, your own human presence). NOT using TP saves weight, garbage, trees... .
When you wash, you are in fact, cleaner.
After a while, you're going to run out of TP anyway. What then?