ST tire brand now a days is a problem. The Maxxis brand has better then most good reviews but they to have had some strikes agasint them. I myself am in the situation of needing new tires due to a tread detachment problem with my Denman's after 3 yeas of service. I now have 2 of them failed within 500 miles of each other on both sides of the camper.
In my case I replaced the 1 failed Denman with a Maxxis ST tire to get me back going again and have a level of trust in the tire. I will also replace the 2nd one that failed on my last trip with a Maxxis. In my case Discount Tire is the place I can get them. I have had 2 sets of Maxxis over the years and both never blew out but had weather checking cracks start at year 4.5. By year 5 they needed to be replaced.
In my specific case I am converting to LT Commercial truck tires. The 5 ST's will move to a flat bed trailer I just bought. Not everyone can do the LT upgrade and it comes with complexity in many cases. Most times new wheels and some times more fender clearance and these issues are too much for many to overcome. So what is one to do??
Here are some thoughts that may help on ST tires that I have collected along the way.
1. Weigh the trailer and split the axles weights on 2 scales or separate so you know each axle weight with the camper as loaded as you will ever tow to camp with it. Include fresh water if you haul to camp towing down the highway. Make sure you have 15% to 20% extra tire capacity over you heaviest loaded tire. Ideally if you can, get individual wheel weights and include the WD hitch hooked up and engaged when doing any tire capacity weighing as the WD adds load to the tires.
2. Check your rims for load and capacity. If they can handle the next load range, B to a C or C to a D, or D to E the added up grade buys more saftey margin. If the wheels will not take that extra load rating, either upgrade the rims or stay the course on the size you have. If you do not have to buy new rims and can upgrade a load range in the same OD tire, this is a big gain and not a lot of money. EDIT for clarity (7-2-12): If you already have 20% reserve capacity over your heaviest "scaled max" tire you have the reserve and may not need the load range upgrade. It you believe your weight will change then it is added insurance to upgrade a load range. The key is to get the extra reserve capacity from known scaled weights on individual tire loads, not estimates.
3. Tire stems. Mandate new high pressure stems. High pressure rubber snap in's or metal stems. They cost squat but can be the difference of night and day in a tire failure.
4. Tire inflation. At the start of every towing day, check the air pressure cold in the morning before towing. Adjust to be spot on the tire pressure listed on the Sunline VIN tag. If you cannot read it then go max cold side wall pressure which is what many of the Sunlines recommend.
5. I agree to balancing trailer wheels. The tires and the axle bearings will ride better and longer. Since trailer wheels are lugcentric, ask your tire shop to balance them on an adapter using the lug holes not the stamped center hole. The center hole does not pilot a hub like on an auto and may not spin true to the tire.
6. Do not tow over the speed rating of what ever brand ST tire you have. Many are 65mph max, some are 60 mph max. Towing over the speed rating creates more heat then the tire can handle and starts the breakdown process ever faster.
7. Most important, at the start of every trip look at the tires. Look for side wall bulges or tread bulges. Do this even at every rest stop. If you spot any abnormality, change the tire then. I got lucky twice now as I spotted the failed tire and changed it before it blew. I know this sounds like there is not a lot of trust for ST tires, however looking and finding a problem before it manifests itself is worth a lot verses dealing with the aftermath of a tire blow out and wheel well damage. Trailer tires in a tandem setting are not the same as autos. They are different.
8. Use tire white covers if you store your camper outside. They do help. I myself also use 303 UV protectant to help ward off weather cracking.
9. 5 years may be all you are going to get from a trailer tire and mileage may not be the factor.
10. Check lug nut torque mandated after the change of a trailer wheel. 25, 50 and 150 miles checks since the wheel went on is a need. Paint in the lug holes and again the lug nuts hold everything on a trailer wheel. They do loosen until full seated. For your Sunline Grade 8 1/2-20 thread studs, 100 ft lbs. is adequate. 120 ft lb is max, 90 ft. lb is min.
11. Dates codes. Understand how to read a DOT dates code and ask for tires ideally no more then 1 year old. You may have to compromise but if you do not ask, you can get anything. Understand the tire warranty. Trailer Warranty
Maxxis tires are only warranted from 5 years from the date of manufacture, not the date of sale. If they sell you a 3 year old tire, you only have 2 years of warranty left.
Hope this helps and good luck