1. When you wash your RV prior to storage be sure to clean the tires. The easiest thing to do is to treat your tires just as you would the side of your RV. Wash the tires with the same soap & water you use on the RV body. You wouldn't use SOS pad on the bodywork so don't use coarse scrubbing material on the tire.
2. Inflate to the pressure shown on the tire sidewall. This will decrease the tendency to flat-spot and make the tire flat on one side.
When I was checking the inflation I discovered one tire was almost 30 psi low. I knew the tire had not lost air when it was being driven so I immediately suspected that when I removed my external TPMS sensor the valve core must not have properly seated. This was confirmed with a shot of household cleaner. Here you can see the small bubbles that confirmed the 3.5 psi per week air loss past the valve.
3. If you are not parking on a concrete floor or pad it is suggested you not park on asphalt as the oils in the tar can attack the tread rubber. It is also suggested that you not park on dirt or sand as the moisture can migrate into the tire and possibly cause corrosion of the steel.
I have some pressure-treated boards that are large enough to completely support the contact footprint.
Here you can see the board and the reference footprint for a front tire.
You always need to completely support the entire footprint. Both in length and width. These pictures show what NOT TO DO.
Be sure the support is wide enough for both duals.
Finally, be sure to cover the tires to protect them from direct sunlight, which can cause UV damage and overheat the tires.
If you haven't read my series on Tire Covers simply check "Tire Covers" on the label list to the left.
A little bit of preventive care can help avoid future problems and extend the life of your tires.
P.S. Don't forget that even when parked you should check the air pressure once a month. If I hadn't checked the air in the tire with the leaking valve it probably would have been flat by next spring and might even have been damaged and unsuitable for highway use.