I always keep oh 10 gallons in the fresh tank. I'm in Cali so I don't worry about freezing. The water is treated so I don't worry about it getting funky for a long time and I believe having some water in there will keep the inside wall from getting funky, does that make sense? Once in a while I drain it and replace the water, but usually we are on city water in the parks so we don't need it. And, I agree, always keep a little in the black and grey so the solids don't hit bottom and harden.
2013 F-250 Lariat 6.7L 4X4 CC short bed
5th wheel prep Reese Elite slider
2013 Denali 280LBS Mountain package