RV Toilet Paper
Fill an empty jar (a quart mayo jar works great) half full or so with water. Put in a sheet or two of your toilet paper. Shake the jar rapidly for a few seconds. If the toilet paper falls apart it's RV holding tank friendly.
ONE PLY OR TWO
Use whichever you prefer. I believe that the folks who prefer 1 ply over 2 probably use pieces that are twice as long, ergo basically the same amount of paper. Can I prove it? No, but I believe it.
Simply put, the brand of wood pulp, water glued toilet paper that people like and suits their budget and is readily water soluble is the brand that they should use. I would also add that if people can abide it they should consider using a brand with a high recycled material content. The only caveat is that the paper should, again, be readily soluble.
What brand(s) would I recommend. Well, that's pretty subjective. If buying in bulk, i.e. by the case, I prefer Scott/Kimberly-Clark 04460. It meets the requirements I've stated above, is a 2 ply, 500 sheet roll, and retails for about 50 cents per roll. It's available at Costco.
Kimberly-Clark 'Surpass' toilet paper works well, too, and contains a lot of recycled material. That's good.
Another brand that we sometimes use is the Kirkland product from Costco. It, too, is 2 ply, 500 sheets per roll and it also retails for about 45 cents per roll. It is not, however, quite as resilient (sturdy) IMO as the other two.
Sam's Club sells Fort James (Fort Howard + James River) 132. It's a little better quality than the KC 04460 but the rolls are smaller (400 sheets per roll) and the price is only just a bit less.
We have found that all of these products dissolve very well in water and in a holding tank environment.
RV toilet paper
I don't believe that a specialty branded RV or marine toilet paper is any better than many store brands, only more expensive. Other folks disagree. I guess that's why there's so many different brands. Each to his/her own.
No matter which brand of toilet paper you use, most of the problems associated with toilet paper clogged dump valves and level sensors in RVs can be eliminated if good dumping techniques are followed.
I bought and use the Flo-Jet Waste Water Pump kit and I LOVE IT! No more waiting at the dump station or paying fees to dump or finding an open dump station. I simply connect the portable pump, run a small diameter hose to my homes sewer clean out and press a button. Problem solved. Read more at: http://www.rverscorner.com/Flojet_RV_Waste_Water_Pump_Kit.html