Roland's question is quite typical during discussions of the use of nitrogen in RV tires. Let's start with the importance of maintaining proper air pressure. It is far more important to keep the proper air pressure in the tires than whether you use nitrogen or air. The air we breath is nearly 80% nitrogen, so adding air to a tire that requires more pressure will not dilute the nitrogen that much.
The process of extracting nitrogen from the air makes it very dry. This is a positive attribute as moisture holds heat and the nitrogen filled tire will cool faster.
Cost and Availability.
The bottom line is that nitrogen is a good product but refrain from buying into the "better mileage" or "longer life" of the tire. Just be sure to always maintain the proper pressure and check it often.