I have access to plenty of metal, stainless and aluminum.. I fabricated this to hold our 10'6" SUP's..
I am NOT sure you will be successful for 12's??
If you have not purchased yet, the new Grand Design is showing a layout that has space for 2 under the rear cargo area, not sure of all the details? That would be a win-win as GD has a very good name here !
Me-Her-the kids
2020 Ford F350 SD 6.7
2020 Redwood 3991RD Garnet