I know there are several places that will custom-make drapes and pleated curtains for RVs.
You don't say what kind of motorhome you have. Are you looking for the type that wrap around to cover the door/passenger window and the driver's side window?
No matter, again, several places will make whatever size you give them measurements to using OEM -type material or something upgraded if you choose to have better material than what you're replacing.
One place I've heard about from two parties that used them (I personally haven't) and were very happy with the product is Gary Manufacturing.
Gary Manufacturing - Custom & Commercial Sewing -- RV Drapes
I'll look in my log to see if I can give you some other choices but I'm not sure if you can order an in-stock product without going to your RV manufacturer which will probably be expensive and could take just as much time.