You might want to read this article concerning tires on 5ver's if you haven't seen it.
http://www.thisoldcampsite.com/This_..._Alignmen.html
I had this problem on my 2006 3400RL. Kept eating tires until I had an alignment. I now have Firestone transforce on and have about 2500mi. without an issue.(knock on wood!) I also had the alignment guy build me a horizontal brace to fit between the Mor Ryde boxes on my 5ver. Mor Ryde sells a light duty brace called "X-Factor". This time if I do have another blowout it's less likely to knock the wheels out of alignment due to the shift in weight from the blown tire.