Bounty hunter is bang on with the leak issue - now to find it!With it being just one corner, and lowers while parked overnight, I'm guessing a bag leak on that corner. Could be a leak leading to the bag.
Pull the wheel, make sure that corner is at right height, and go over the entire system with a spray bottle of soapy water to look for the leak.
You need to spray the fittings of the tubing at both ends. The other issue it is pin hole leak in the air bladder it self very difficult to detect with out a pressure check.
Hopefully you find the leak at a fitting - easier and cheaper fix.