I seriously doubt the diaphragms are simply vibrating it to 100psi.
- Hot water tank overheating and building pressure could be an issue. I don't recall the pressure when the hot water pressure relief valve will release but might be around 100psi.
- Unlikely but the water pump has a spring loaded pressure switch. If the spring is weak, it could be turning on intermittently while bouncing down the road...but as mentioned, the odds of getting two unit with this problem is highly unlikely.
- A bad pressure gauge may be the problem.
Personally, we usually turn off the pump and water heater while traveling. If we stop for lunch, it takes just a second to turn on the pump and the hot water heater will stay hot for several hours, so no real burden. This avoids the potential for a leak to spray the entire contents of the fresh water tank before we find out and/or burning out the water pump.