Trailer Tires

How to extend the service life of trailer tires? I have a tandem axle 20' open car trailer. I have owned the trailer since 2006 and have replaced two pairs of tires since I owned it. One pair in '07 due to road debris taking out 1 tire. In '12 I replaced the other pair. Yesterday I was checking out the trailer in preparation for this trailering season and noticed that both pairs seem to have some dry rot cracking. The cracking is all in the tread area, not the sidewalls. I expect that from the 7 year old tires but the 2 year old tires don't seem far behind in the looking sketchy department. The '12 tires were bought at a discount online outfit. Did I screw myself by saving $20 and does anyone have a brand they have good luck with? Also is there anything that can be done to help preserve them when sitting? I use the trailer about 6-10 times per year and typically long distances, 2-8 hours. I notice camper trailers parked with skirts over the tires, does that really help?

I have a set of these on my trailer tires: http://www.amazon.com/Wheel-Covers-Trailer-Diameter-Tires/dp/B0046I14RA

My theory is that UV damage causes the cracking, so keeping the sun off them should prolong their life. Mine are only 2 years old, but still look like new.

I convinced the wife that I was willing to sacrifice and get a full-on garage instead of just an outside parking place at the storage lot so we could store Christmas decorations and baby toys and stuff. Bought a brand new set of tires for the trailer in 2011, put about 7000 or so miles on them since and they still look brand-new.

Park it inside or get good, heavy covers for the tires. Everybody I've talked to says that the tires on trailers will almost always dry rot from UV exposure before the tread goes.

In further thinking about where the cracking is, I'm not sure the sunlight theory is the answer. One side is parked next to a berm 3' tall and is constantly in the shade. The other side does get sun, but the cracking is in the treads, not the sidewalls and the spare, also '07 looks fine, it is in the sun more than the sunny side. I'm wondering if I don't run the correct tire pressures. The sidewalls say 50 PSI max for max load. My car is fairly light, 2650# and I usually run 45 PSI. Will have to consult the tire shop when I get the next set.

I don't understand the whole "UV" damage thing. Why wouldn't car tires do the same thing?

Its to my understanding most trailer tires today are E36 M3, and you're best off with LT truck/van tires.