I just learned something new with the dual
propane tank system on our 1988
Scamp (we do have the new style tanks). I finally got the water system working on the
Scamp this summer, but never fired up the hot-water heater. Since we only used the
propane to cook, I removed one of the
propane tanks to save
weight (plus, we didn't even make a dent in one tank all summer).
I assumed that the pressure regulators that were on the trailer worked as one-way valves and would automatically prevent propane from the other tank from escaping. I had capped the line on the tank that I removed, but it was not a super tight seal. A little while ago, I tried firing up the heater and the water heater and couldn't get them to
light. So I took it to the local RV shop and had them go through it.
Turns out that there was some propane leaking out the capped valve. It allowed enough pressure to build in the system for the stove to work fine, but not for the heater or the water heater.

Thankfully the leak was outside and never allowed enough gas to accumulate to be dangerous (or even smelled). After hooking up the second tank, everything worked perfectly.
I guess some of the newer RV propane systems do automatically shut-off one side if the tank is removed, but many others don't.
So, if your system is setup for two tanks, be sure to have the correct safety precautions in place if you decide to run the system with one tank removed. I'd hate to see someone get injured.