[onewhippedpuppy]

Many years ago I did maintenance for a mobile home park, this happened a lot because many older mobile homes are poorly insulated underneath. The PEX might be ok, I've seen it swell significantly then return to normal shape with no leaks. I strongly suspect you will have leaks with the copper. As the other guys said, thaw the place out then turn the water on slowly with multiple people watching for leaks. Make sure you have someone at the valve to shut it off quickly as leaks are found. You will likely chase leaks for a while, i.e. you fix one and find the next one. It sucks, good luck!

Edit: one more thought, if you find multiple leaks it might be easier in the long run to just replumb the entire place. Otherwise you could be chasing leaks (and risking future water damage) for a long time.