I agree - Jayco is just playing it safe. Like any manufacturer, they do not want to put themselves in a position of approving a modification to their product that they cannot see or control. As long as the upgraded tire fits in the wheel well with adequate clearance, the rim is rated to handle the max tire pressure listed on the tire, and the unit weight does not exceed the axle capacity, you should be good to go. Our 5er came with D rated rims and tires, but the previous owner (original) upgraded to E rims and tires for a greater margin of safety, and the rig is now 10 years old with no damage or problems due to the upgrade.
2003 Ford F-350 V-10 Crew Cab 4WD Long Bed
2004 Jayco Designer Medallion 29 RLTS 5th wheel
Bill, Gayle, and Mindi (dachshund)
Retired at last !
Nothing is foolproof to a sufficiently talented fool.