It seems logical. But nothing I've read or seen definitively says that a higher load range will give more longevity. All I've ever used is load range C and I have never really had any problems. I've gotten 6 years on my tires on previous trailers and only replaced due to age. One set had well over 15k miles on them. The key is proper inflation, staying within the weight rating, and driving at or below the speed rating. 5 years is actually just about right. Depending on the manufacturer, recommendations are to replace ST tires every 4-7 years regardless of their physical appearance. Its also not uncommon to see tires on new trailers that are already several years old. Its not right, but...
That reminds me, I should look at the date stamp on mine...
2013 Jayco Jayfeather X20 E, 2008 GMC Sierra 1500 Crew Cab Z71 SLE