While I've never really had a problem with the holdings tanks using water only, on longer stays when it's hot out i do add a packet of the blue powder I keep on hand.We get about two weeks of black tank use if the men go out side and let the women use the facilities. If we're staying longer than two weeks, we use a porta-potti.
I also use it in the porta-potti as it makes it much more pleasant to empty as we use a porta-potti a lot, especially in the winter.
Water is the key for sure and I'm one that leaves the black tank open in freezing temps and never had build up. Everything in rving is a learning experience and as the years go by we all come up with little things that work for us that we have sometimes learned the hard way.