The only RV that I could completely empy any tank on was one that had access to the bottom of the tanks and I could push up on the actual tank and reverse the sag.
We always leave a little water in all our waste tanks, but it would be nice to be able to actually drain the fresh completely by using the drain valve. But, evidently it is just not meant to be.
Before we retired, when we couldn't use the RV as often, I drained the fresh tank and ran the pump until no water came out. Then I raised and lowered sides/front of the RV to get more out and left the drain valve open. 5 months later while taking the 'dry' RV to a CG, water was still pouring out the fresh drain valve while going down the road
So now I just do as mentioned earlier and sanitize the fresh tank periodically.
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Scott and Liz - Southern NM
2012 Wildcat Sterling 32RL - w/level up (best option ever)
2007 Chevy 2500HD Duramax
Reese Fifth Airborne Sidewinder