If the trip is a weekend trip, especially in cooler weather, I would not worry about any tank additive. We use happy camper additive and have been really happy with it, but like I said a weekend trip in cool weather I wouldn’t add anything. Some of the additives are pretty nasty as far as chemicals and we haven’t seen the need for more than happy camper even on hot summer trips. Happy camper is a favorite here.
As far as adding water, none needs to go in the gray. The best practice is to put it in the black. 5 gallons, or so, is ideal. You need water in there to keep the stuff from clinging to the bottom of the tank and to get a good flush of the tank. I don’t put much water in the black, but it is better to do it. If we are moving I’ve found that the rate of water accumulation in the black is fine that I don’t have to add, but sitting it needs a good shot of water to start.
Keeping the black tank clean is pretty important since things can build up if you aren’t paying attention. If you don’t have a tank flusher installed, you can put water directly in the toilet after a dump and dump again. Better is a Valterra hydroflush IF you have a straight shot into the black tank. It works great on our tank. The hydroflush can actually completely fill our tank if left on with good water pressure. Couple rinses and the tank water is clear.
We don’t worry about toilet paper brand, but things like paper towels are pretty bad and can cause a clog.
2006 Sunline T-1950
2019 Ford F-250