ST tires are supposedly forumlated rubber with more ozone protection and belts better able to resist shifting or separation from squirm typically seen in back in and sharp corners.
I would think they would last a lot longer than five years.
Our G670s, which has faults,on Mh are supposed to last ten years. Ours on the rear are eleven yeas old. I'm going to replace them but they are still supple, soft and no checking or any indications of age. keep dirt off the insides too as dirt sucks oil out of rubber.
Michelins are rated for five years on our MH steer axle. I'm going to try to get a little longer but will monitor condition.
The Michelins on my F150 are six or seven years old and no checking. It is driven often with 197,000 miles on it.
We had Goodyear Wranglers, on our fifth wheel, from the factory. They didn't last a year and had some damage from one throwing the tread.
I think I can recomend Michelin light truck tires, with suitable higher weight rating than the ones on yours from the factory. May have to change rims in some instances because many rims can't take the higher inflation pressures for long.
If you do get aluminum high pressure rims rated for the tire and rv weight.
We've had many sets of Michelins on fifth wheel and three duallies, this F150, cars and others. Just make sure it has the proper load range. I would go one higher than what cam on it. Expect to use more tires if it is a long fifth wheel and gets a lot of jack knifing backing in or jsut normal use. Two axles or three trying to scoot around will make belts shift and delaminate.
jackknifing in back in does better on tires on wet pavement and dirt or grass.