My 5er is 10.5k unloaded and had 225/75/15" D rated and load cap. of 2540 per tire. I just installed 235/85R16 E rated and load cap. of 3042 per tire. How can manufacturer get by with installing a tire set that is overloaded or near overloaded when built? I needed to raise my trailer anyway because my truck with high hitch made the trailer a little off level when towing. Note size difference with tires. Camper photo is with old tires. Fender flares will have to be trimmed slightly.

http://www.gorowan.com/2004ram/DSC_1299.JPG

http://www.gorowan.com/2004ram/newtires.jpg

http://www.gorowan.com/2004ram/sidetire.jpg

my guess would be first cost and second tires should be close to limits of the weight rating on the axles. I would check that next.... some manufacturers are very close to axle weight limits at dry weight and rather than moving up to the next size axle, they save them money. friend of mine had this problem but dealer/manuf would do nothing.

16" are much better for trailers, you won't have any blowouts now IMO. I see many with the 15"

Fifth wheels often have a set of OEM tires that are not sufficient for the entire weight of the coach. Since the full weight of the 5er is never supported by the tires/rims/axles, the manufacturer's can get away with this. The truck carries around 20% of the weight when towing and the front jacks support at least that amount when parked. Our previous 5er only had 15" load range C tires OEM, so I switched to 15" load range D just to have a little more margin. Not all RV makers cheap out quite so much though.... Our current 5er came straight from the factory with LT235/85R16E tires, so the 4 tires' combined weight rating is approximately the same as the total 12,250 pound GVWR of the coach.