You are doing all the right things to extend the life of your tires. There is no set age when tires need to come out of service. Commercial truck tire casings that are retreaded over and over again can often be well over 10 years old when they finally reach the scrap pile. So, if there are no visible signs of cracking on the sidewalls from exposure to UV, you should be fine. If you have any doubts, take the RV to a tire dealer you know and trust for a firsthand look and some expert advse.

If you look at popularity (and availablity), 22.5 is the way to go. If you look at fuel economy, I think 22.5 is the way to go, also. In the scrap haul business with a day cab type tractor, I'm doubting that tires can have a measurable effect on fuel economy. Tires play a bigger role when trucks run long hours at steady speeds. Start and stop operations minimize the impact of the tire portion of the fuel usage equation.

At pressures around 100psi, a 10 psi change in inflation pressure will result in about a 1% change in fuel economy. Obviously, higher inflation pressures result is lower rolling resistance and less fuel usage. If the tires and wheels you are running are both rated to a max pressure of at least 110 psi, it is OK to run the higher presssure. But, the ride in the vehicle will be more harsh and your tire wear might suffer if your tires do not need that much pressure to carry the loads they normally carry. In general, the positives of having too much pressure typically outweighs the negatives.

If I were in your shoes I would do the following: Get a tread depth gage and check each tire for remaining tread depth. Check the depth at 4 places around the tire, 90 degrees apart. Measure at the lowest reading on each part of the tire (inside, outside, center of the tread or anyplace the tread is low for any particualar reason. While measureing the tread, look for objects imbedded in the tread that may have been picked up when traveling downt the road. Inspect the sidewalls for cuts, scrapes and cracking due to age and exposure to sun. In my case, I would make a judgement on which tires were best and which tires were worst and replace the worse ones. But, I have been envolved with tires for over 37 years. You might want to strongly consider visiting a dealer that you trust to give your tires a similar inspection and use his experience to make similar judgements.