I still suspect valve stems as the likely problem. The nitrogen suggestion by the dealer is somewhere between idiotic and moronic. Air is almost 80% nitrogen.
Valve stems are an inexpensive fix. Buy 4 stems and an extractor tool at a local car parts store. You must have a compressor to fill the tires, though. While you could pay someone to do this for you, it will cost a lot more than the 4 stems, the extractor tool, and the 120V compressor will. And then you will be able to fill your tires with air any Saturday morning you wish to. One more thing: you need a tire pressure gauge. You can get a great dial gauge at Sears for less than $10.