I've not heard of any such product and not even sure how it could be employed. Moisture, if any, will be in the bottom of the tank but occasionally get sucked up into the supply line and block regulator or a gas burner orifice. The usual method is to pump the tank out completely, remove the valve assembly, and physically dry it out. A pro may also use anhydrous methyl alcohol in the tank to absorb water, but you still have to open up the tank to use it. There is no way to just pump it in while adding LP, like you would with a car fuel tank.
I would be rather surprised if you have a water problem in a 2013 RV. It mostly occurs on older rigs that have been refilled numerous times, or let sit with near-empty tanks over many winters (condensation build-up). Maybe you got a bad fill of LP somewhere, but that would be well down on my list of possibilities. I gather your regulators are clogging, and replacing them solves the problem for a time? May be oil or other sediment in the tank, but the solution is still the same. If that's your problem, you need a professional LP service, not an RV dealer.
http://www.ehow.com/how_12064514_remove-water-propane-tank.html