Re: Load range...C or D?
I don't trust the cracks you have, especially on on summer days out on the Interstate. They can blow out. Radials are nice, but the squatty side wall sticks out and rubs curbs more than bias. I don't know why some dry rot and some don't, cheap rubber I guess.
A lot depends on frequency of use and how much sacrificial agents the tire manufacturer used in the tire's construction.
The two major degrading agents tires face are, as noted previously, UV and ozone. Tire manufacturers combat this by adding sacrificial agents to the tire/rubber compound. For UV, carbon black is the choice. For ozone, typically a type of wax is chosen. These act like the sacrificial anode on an outboard in giving the corrosive elements something to more easily attack than the actual rubber.
But, as these sacrificial agents are used up/attacked by the respective destructive force(s), the outer wall has to flex to replenish the levels of carbon black and chosen wax to keep the sidewall from drying out and cracking. Car tires typically see lots of miles compared to trailer tires, so car tires are constantly flexing and replenishing the outer skin of the sidewall with the sacrificial substances.
But trailer tires, due to them being used comparatively rarely, can and do have their outer skins of the sidewalls completely use up their sacrificial substances and consequently the UV and ozone begin to attack the rubber, causing the dreaded "dry rot" symptoms....cracking, bubbling, hardening of the tread, etc.
So, unless you plan to use the tires/trailer more to flex the sidewalls more, we're down to covering them to slow the degrading of the sidewall rubber.