Defective batch of fittings... maybe. But I think there most likely is an entirely different cause. As a reasonably adapt Mechanical Engineer, I can think of at least one other cause for several trailers to have the same problem. And, I can also think of a reason that others with the same batch of parts would not.
I have hull 342. When I picked up my Ollie II, OTT provided a small brass "Flow Restriction Device" (FRD). I would wager that #381 and #390, and #412 as well as a whole bunch of other hull numbers have the same device. I suspect that some of those that have had this problem were using the OTT FRD at a trailer park with very high city water pressure when the problem occurred.
If you look at the OTT provided FRD, it simply is a small orifice between a male and female hose bib fittings. This is NOT a pressure regulating valve (PRV). WHY? Because such a device under no flow conditions will sill allow line pressure at the faucet to be seen inside the trailer. Granted as you open a faucet in the trailer, the device will restrict flow and as such the pressure inside the trailer will also be reduced due to having an open valve. But if you have 125 PSI at the camp ground water source, and are not using any water in the trailer, the pressure inside the trailer will likewise be 125 PSI.
IMHO, the OTT provided FRD is a piece of junk. Using it is tantamount to playing Russian Roulette with your plumbing system. The only use I can think of for this device is to use it as a trout line weight. If you have one of the OTT provided FRD's, I STRONGLY recommend you do one of two things.
1. Buy a real Pressure Regulating Valve (PRV) such as the one like John D. suggested. (See picture below that he posted).
2. Don't use the city water connection. Many OTT owners use the fresh water fill port to fill our fresh water tank and we use our pump to supply our needs. Since we don't use the city water port, we have not blown out our plumbing.
I recommend that OTT put out a TSB to have all OTT owners trash their FRD's and to purchase a PRV OR, not to use the city water connection.
I also recommend that OTT not provide new owners with FRD and give them real PRV's.