I would try a straight spring time de-winterization steralization
1/4 cup bleach per 15 gallons of water.. Flush all lines. Drain
then Baking soda. as much as you can get to disslove well flush all lines, drain.
THen clear water. Flush all lines
If you go heavy on the soda and a little remains it will not hurt you , you won't likely taste it. and it may help if the multiple rinses does not get it all out.
As some said you may need a new ater pump.
How anyone can mistake a water cap for a gas cap I have not clue. they are nothing alike and the water cap is often locked behind a door.. THough I was just looking at a motor hom (Afrin? By Tiffin where I commented to the owner that the water fill access often took the same key for nearly every motor home. and then proved it by taking out MY key and opening the door.. and 2 more
ONE was the door behind which the gas cap hides.. all 3 took same key as do the two such panels on my Ride.
THen re-fill