I agree with 2oldman, it sounds like a lack of water problem rather than the TP being used. We've used Charmin Ultra Soft for many years in multiple RV's with no problems. Pretty much any TP labeled "Septic tank safe" or similar should work fine with enough flush water. That said, we also have a third twist-on valve in place that we use to either back flush the black tank just for more thorough cleaning or to allow the tanks to equalize when we need additional capacity when no sewer hook up is available. It's usually the grey tank that needs the extra capacity, so the flow is normally from the grey to black in that situation. I don't have any concerns with the two tanks "cross contaminating" though, since it's all sewage that's going to end up in the same place.
