Consumer's Union, publisher of Consumer's Reports, tested TP several years ago and found some significant differences in the speed of dissolving. I don't recall which brands were the fastest. All eventually dissolved.
But, our RVs have holding tanks, not septic tanks. We dump our holding tanks into sewer systems, and most (if not all, can handle the slowest dissolving paper. It's the fat and grease that give the systems problems.
We use the same stuff we use in our two stick houses, Charmin, and have never had a problem in over 20 years of RVing. (One of the stick houses has a septic system, but the tank had very little "ash" when last pumped. Ash is what the pump guy called the solid waste at the bottom of the tank.)