Our 5th wheel Toy Hauler is about 14k # when loaded up. The Truck is an '06 F-350 CC LB 6.0 with the OEM 20" Ford aluminum wheels and Goodyear Wrangler AT/s tires.
I know the 20" wheels are not the best for towing. Also I've had TWO wheels crack and TWO blowouts. I'm not pressing my luck any longer with these wheels and replacing with 18".
You can check out the cracked wheel post here.... http://www.ford-trucks.com/forums/10...l-cracked.html
The Goodyear AT/s are the OEM tire and I've always just replaced them. I'm on my 5th set in 175,000 miles. Time for a change as well. Haven't had good wear out of the Goodyears along with 2 front blowouts.
I was thinking of trying the Michelin LTX AT/2's E rated. They get high reviews. Just don't know how they handle towing heavy.
I replaced my stock Conticraps about 10K miles ago with Michelin LTX M/S-2's on my factory 18" rims. Love the new tires in every way. With the 70K mile guarantee, you can't go wrong. My F-in-L got the ATs on his 2012 SD and I swear they're already at 50% and he doesn't tow anything. I've never had them, but I'm not so sure they'll hold up very well with a heavy 5th wheel.
How big is your trailer, and do you have a verifiable pin weight for it- not a guesstimate. If you're cracking wheels and blowing tires, I suspect its considerably heavier than you think it is. I have 18x8 factory rims and am running Toyo Open Country A/T's size LT 285/75R18E rated at 4040lb. per tire. I have a 2750 lb. pin plus usually haul 10 gal extra fuel and typical camping crap/tools in the back and these tires don't even get noticeably warm. Zero problems with them and good tread life. I have another set in M/T tread for the winter.
Just my $.02, but I agree with what onug said above, if you are going with Michelin stick with the LTX M/S-2's. I had the original LTX M/S on my 1999 F-350, 265/75-16's and they worked great pulling my 40' 16k# toyhauler. The problem with the Michelins is they start cracking before you can wear them out, so mine still had about 10% tread left when I replaced them with the same size LTX-A/T-2. These tires drive great empty, but when I hooked up the trailer they had so much tread squirm I pulled over because I thought I was getting a flat. It was a white knuckle ride the whole way.
Luckily (?) these tires wear really fast, so after a couple thousand miles everything was back to normal. I doubt these will make it 30,000 miles towing, and when pulling a grade you can just hear the rubber shearing off them. On the other hand the year I got them we had more snow than I've ever seen here before, and they were unstopable, I was plowing through the stuff like it was nothing.
But next time I will go back to the LTX M/S-2's for sure.