Are the tires covered to protect them from UV? That will have a great effect on the life of the sidewalls. For all intents I would probably say that once you hit the end of year 4, you probably will have to change tires if you start towing again. Whether you are down on the ground or up in the air the tires will deterioriate. Obviously down on the ground I also would be concerned about the belts, so just figure if you are going to tow you will have to replace the tires. Getting you up off the ground at this point won't give you much more life if you plan to tow.
I don't know the layout of your site but I'm surprised that you can't move the trailer a little (at least twice a year). It doesn't take much distance to get a quarter turn on the tread.
'04 Classic 30' S.O.,'03 GMC Savana 2500,'14 Honda CTX 700