If you put water in the tank and immediately drain...and you hear water flow out, the valves are working.
One possibility, do you have a leak? We had a cracked drain fitting on a prior trailer and the water would all leak out before we went to drain. Only figured it out when I went to drain as tammy finished dishes and I saw it leaking. It was just a drip but over a few hours, it could empty the tank.
No a chemical addative is unlikely to do anything since you haven't put in any solids (not that it's likely to help even if there were solids).