I've done all three permutations over the years:
1. air only
2. air then red pop (RV antifreeze).
3. red pop only.
I did air only and the in-line filter cracked on me and I had water all through the trailer when I filled up at the start of my first camping trip this year. @#!%^&*. This year I'm returning to No. 3. Red pop "injected" ahead of the filter, pumped until it comes out the faucets. That will take less than a gallon, even if there is plain water in the pipes at the start. The remainder split between the sink traps and the toilet.
By "injected" I mean that I put a tap into the water line between the tank and the in-line filter and I suck the red pop out of the jug so that it goes through the in-line filter and the water pump before it goes on to the pipes in general, hot water tank bypassed.