Another reason for keeping the fresh tank that I did not see mentioned.
Went to a full hook up park in late Sept. Everything was fine and working with my full hook up but a cold front moved in. Temps got down to the low 20s and city water connection froze. I had to deal with that situation by disconnecting the city water connection and filling my fresh tank as temps were going to be down in the 20s again the next night. With my fresh tank filled, I was able to use water as needed even with my full hook ups at the RV park.
This may not be a common occurrence, but I'm very glad I had the ability to still have water with those cold low temps.