Tire not holding pressure

I checked the tire pressure this morning (after getting it to 60 psi last night) and it was back down to 40 psi. But the mystery has been solved so it is time to bring closure to this thread. I would like to thank everyone who commented. No single poster had the answer that solved the problem, but by synthesizing the various responses, I was able to formulate and execute an action plan. Best response goes to Bob_nestor who suggested I take it to McKinney Tire and Appliance on McDonald Street. Well, I almost did that, but the reviews of that establishment is enough to give pause. I did take it to Thomason Tire just down the street in McKinney. They had the coach in at 8:40 a.m. and out at 9:00 a.m. with the problem solved. Dead center of the tire was an embedded self-tapping screw. RVs seem to be rolling magnets for those things. They pulled it, repaired the leak and all is well. Here is the kicker: total cost $15.00. I had to have the guy repeat that to make sure I was hearing that right. Sure beats $150 for a heavy duty jack from Northern Freight, $29.99 for a Kiddie pool, $8.00 for a bottle of Goo Gone, 2 hours of my time and $500 for a trip to the emergency room for a severed thumb. :-)