One of the new tires on the fifth wheel was down to about 56# this morning, so we took the fifth wheel back down to the tire shop to have it checked out. The guy couldn't find anything wrong with it, so we all came to the conclusion that it's a defective Pressure Pro sending unit that is causing the tire to loose air. We'll keep it off for a few days to see what happens. If the tire doesn't loose any air, we'll have to get a new sending unit for that tire.