Larry -
I also experienced the same dragging rear end on my Coachmen on occation. The over hang will be slightly less on the conversion, and it has not been a issue that Show Hauler has encountered.
As for the OSB structure board Show Hauler uses. OSB by design has much more adhesive within it than does plywood. So it actually resists water penetration better than plywood. Although once it does get wet from a presistant leak it does tend to degrade faster than plywood. OSB has almost a 2 to 1 shear strength advantage over plywood, and I would say the most important advantage is it's 1/3 cost savings over plywood. Since this is one of those overkill areas Show Hauler preforms, they try also to keep costs as low as possible to there customers.
2003 28' Show Hauler Motorhome on a 1995 FL 120 completely refurbished chassis.
Currently in production - scheduled completion 12/10/02 www.showhauler.com