I wonder what's really wrong with using a spare to run on after it has been stored in or on your RV for years if kept out of the elements ... especially out of the sun? Our Class C's spare is way up high between the frame members in the rear right aft of the gas tank.
What's the difference between the situation above and the situation of using a tire for years and years in a running position on a vehicle if the tread is still fairly deep? I've had one tire set on my 1995 4X4 pickup for well over 12 years in all 4 running positions - and the pickup sat out in the weather and part-time in indirect sun. However, their sidewalls never showed any sign of cracking, unlike some folks see in their old RV tires.
Tire safety is of course high priority - but sometimes tire usage fears may be overblown with respect to high quality tires with good tread that are kept properly (not too soft) inflated and out of constant direct sun. IMHO, hour after hour of direct sun exposure and excessive heat from too-low tire pressures while driving are the main culprits in the weakening of tires.
Keep your tires cool when rolling and away from the ultra-violet rays when sitting.
2005 E450 Itasca 24V Class C