As usual, I was quite glad to see my Motorhome
magazine today, and one of the first columns I read is "Quick tips from our Readers" because there are a lot of good ideas, which you say, "why didn't I think of that." "It is so simple."
Well, I had the same problem as James Briner (from Bradenton, Fld) who wrote this quick tip. I was loosing 10 to 15 gal. of water from our tank through our vent, and since we primarily camp in state parks, we need all the water we can get. So, like he suggested, I installed a ball valve in the vent line, and I did not open the valve until we were set up and level, and we quit loosing water. I've had the valve for 4 1/2 years.
Last summer, probably on a 3 day holiday, I was blocking the camp loop, while I was getting water, and of course was in a rush. After about 15 minutes I heard a loud explosion, really like a cherry bomb, and I started loosing massive amounts of water out of the bottom of the RV.
Now, I don't have to tell you what happen. Normal water pressure is about 40 PSI and with no air vent, well....... $3,200 to fix. Back cap had to come off, sub flooring had to be replaced, and the sad part is I went from having a 75 gal. capacity to a 58 gal. capacity, because the tank that (the now out of business National) used, was no place to be found.
Yes, I still have the valve, but I don't think I will ever forget to open or close it. There can be just as much of a problem, if once you get level, and you do not open the valve and you continue to draw water from the tank, ..... I venture to guess that when you draw out 50% to 60%, the tank is probably going to split, or if you are real lucky a hose will give way, but the vinyl hoses that are typically used will withstand quite a lot of pressure/vacuum.
The valve is a great idea, but NEVER, NEVER forget where it is set when you are filling your tank or using water from your tank.