At work, we have a 1-ton dually that is used as a work-horse to move heavy equipment trailers and generators. It's a 2014 model and only has about 8K miles on it. Obviously, it sits the majority of the time. We just replaced the tires due to weather-checking/sidewall-cracking from sitting int he sunlight. The same brand/model/size tires on a daily-use vehicle are still looking great with nearly 35K miles on them. Sunlight will KILL a set of tires if they don't get used often, as in the case of most RVs. As a result of having to pay out $1,200 for a replacement set of tires on that truck, I bought tire covers for ALL of my trailers and equipment that sit in the sun for longer periods of time.
So yeah, I'd say the covers are worth it. Covers cost between $30-$60/set of 4.....tires, around $400 each.....not to mention reducing the chance of a blowout as a result of cracked sidewalls.....