If after a spin test, checking for out of balance condition, and replacement - if it happens again, check if your rig is out of alignment. We had that condition for two years in a row. Our RV tech recommended a local shop to check alignment after replacing the hangars, U bolts, and replacing the Equa Flex suspension parts; he was right. We were out by 1/4" and that's a lot. The alignment check was about $250 but well worth it.
Every year we take our FW to the tire shop and have them to a spin test and inspect all the tires. So far so good after new tires in 2014 and fixing the above issues.