fresh water tank overflow syphoning tank dry

Lynnmor wrote:

FLY 4 FUN wrote:
I find this thread interesting. Our fifth wheel also has a pressure fill and valves to determine city water/system use/winterize etc. Last year I started noticing that when I fill the tank to capacity it would drain WAY longer than it used to when tap shut off at house. I would get to campsite and notice my "idiot light" was showing 2/3 tank. Not trusting it I ignored it. Now I show up at a campsite and find I can add 1 or 2 6 gallon jerry cans to what I believed was a full tank. In addition our family following us on road trips notes that we lose water on left turns and not just a little...looked like smoke coming off the tires. The last time I topped off the tank I noted water coming out overflow but when I got under the rig there is actually two red "overflow" tubes with one coming out hard/fast and the other trickling out. I wonder why there is two tubes? Is one an overflow and the other the breather tube? I would really like to understand them and may have to pull the coroplast to see whats changed as it used to just drain off a gallon or two as the tank was full and somewhat pressurized.

There are two VENT tubes to prevent pressurizing the fresh water tank. With a connection to city water, the incoming pressure can be quite high and the RV manufacturer does not supply a regulator. Two vents give greater capacity for out flowing air and also adds redundancy in case of one becoming restricted. NEVER add valves to these vents, if you ever forget to open them when filling, the tank will expand causing considerable damage. If you pump water from an unvented tank it will collapse and possibly dislodge from the supports. If your check valve in the water pump or the fill valve leaks, that too will fill the tank when connected to city water so the vents need to work always.

The fact that you didn't see the water spilling out in the past was just a matter of chance, straight roads cause less spilling.

The answer is to run the vents up and out of the coach well above the water level.

I added a gravity fill port of the hatch type. The elbows at the top are the required vents. The small holes in the door act as an insect screen and allows venting with the door closed and locked. The fill hose was attached to the tank by spin welding a hose barb to it.

I don't disagree that running the vents up and out is a good solution. However accessing the tanks to accomplish this is not a simple task.
Putting on valves is very easy. Tie a string to your faucet/finger or whatever it takes to remember to open the valves!