Dan,
Tom and Mango are both right. There are two ways to deal with a leaky tire valve. The first is to replace it. Sometimes this is the only option, but the second way often works and that is to let the tire pressure bleed down a little so the air flow going back into the tire has a much higher pressure than the internal tire pressure. Sometimes this literally blows the offending piece of grit from under the valve seat. If the leak persists you do have to change the core, which is no big deal on all but the inner tires.
Bill,
I don't know why, but colder temperatures will cause leaks in the air system show up sooner. As time passes the leaks causing the more frequent compressor run time will be showing up in the warmer temperatures. Enjoy what now sounds like a very air tight coach.