I didn't believe it until I saw it on my trailer. A bit of history:
We had a Casita with only 25 gallons of water and no overflows other than the vent built into the fill opening. Nice thing was it was easy to actually look at the tank and know exactly how much we had left. Typically on a 3 day weekend, wife could do a marine shower on the 2nd or 3rd day and we'd still leave with water.
Flash forward to our upgraded trailer in my sig. It's got 55 gallons of water, which was a selling point for me as many in its size only have 30. Anyway, things were going pretty good - as we conserve we didn't tap out. Then we had a long weekend where we ran out of water. I couldn't believe, how we use water, that we ran out and assumed a kid had done something to waste it all away. However, the holding tanks didn't confirm this so I knew something was up.
At this point I read some thread on forums about the "siphoning" including the tank collapsing because of it. I wrote it off as internet jibberish.
Then I got home and started testing.
Started filling the fresh water tank. It takes a while. Then, eventually, I get a small trickle out of the overflow tubes. A few seconds later, I get a hard steady stream out of the overflows. I can remove the fill hose right at that point, and the overflows will continue to run for a minute or two. It has to be 10 gallons or more that pour out, almost under pressure, when this happens. Then they stop very suddenly.
I have to assume that same siphon type activity could occur while on the road with a good slosh. The tank itself has to be flexing/compressing - there really is no other explanation I can think of.
I went to Home Depot and found a couple of valves I then placed on the overflow valves. I leave them open when filling until the siphone effect starts and then close them and shut off the water.
After I installed the valves it made me wonder how the trailer was plumbed. I decided to close the valves and fill as full as it would go. I learned that the air valve in the fill hole is plumbed in, however the fill hose on my trailer, when full, will leak.
Anyway, we had a good 4 day weekend since then. It was hot - 100 each day so that uses water. Yes I drink my tank water, but that isn't my primary water source for drinking when camping like this. Anyway, we used water as normal - IE: conservatively - and when we got home had 25 gallons + left in the tank (tested with a 5 gallon bucket under the drain valve)
Around $6 at hardware store spend - water capacity greatly improved - happy camper!
PS: I'll update my blog with this post - modified with pics and maybe even vids.