Interesting problem with putting pressure into a rectangular prism - it tries to convert itself into a sphere.
Take a typical example of a RV tank say 50" x 30" on the flat side.
Imagine hooking the mains supply up to the tank and the tank has only a piddling 1/2" vent pipe. 100psi water at high flow rate blasting into the tank and trying to get out that small hole. Say the pressure inside the tank rises to 20psi.
The force generated upwards and downwards causing those flat sides to bulge - say 3" up and 3" down is 50 x 30 x 20 = 30,000 pounds
Damn near lift a whole class A off the ground all four wheels at once.
Sure, the mains pressure reducer should stop the pressure getting that high, but do the sums for even 10 psi and it runs into big numbers and big forces.
Even poking the hose down the gravity fill inlet - right down into the tank - to stop "burping" - and then turn the tap on full - can easily result in 4 or 5 psi in the tank and that will also bust restraining straps and even hardware store bolts.
Some fill systems use a restriction in the inlet line to keep pressure rise under control (and the regular RV inlet regulators include that), but lots of RV owners have nothing.
If you break mountings, it may not be the fault of the design
Tony Lee - International Grey Nomad. Picasa Album
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RVs. USA - Airstream Cutter; in Australia - MC8 40' DIY Coach conversion & OKA 4x4 MH; in Germany - Hobby Class C; in S America - F350 with 2500 10.6 Bigfoot camper