Hi Newcamper!
RV mattresses are always a pain when it comes to linens. With our latest RV mattress, I was able to solve the problem at Bed, Bath, and Beyond. They have queen sheets that fit the newer extra thick (up to 15 inches thick) mattresses. The sheets I used on our previous MH's mattress, which was the same size but thinner, weren't large enough because of the added thickness of the newer mattress so you have not only a size issue, but also a thickness issue. Yes, I suppose linens might cost a little more at BB&B, but you'll have them for a long time and they're decent quality. They also have mattress pads for the thicker mattresses. Because our bed platform lifts up for storage access, they can't make the platform as long as the mattress or it would hit the ceiling so blankets are an additional problem. Regular queen blankets fold under the mattress but come right to the edge of the platform and cannot be folded under between the platform and the mattress. To solve that problem I took about two feet of an old sheet, edged it, and sewed a new bottom to the blanket. I've found that the extension works better if you make it the width of the bed platform rather than the full width of the blanket. Also, because it's under the mattress it doesn't show so still looks okay.
Good luck on solving your problem. Remember, where there's a will, there's a way.
ArdraF