I have heard the Flexsteel supplied the Roadtrek front seats a couple of places, but nowhere have I ever been able to find any reference on an official Roadtrek document or website. Only second hand information. A Roadtrek sales guy at a show back then was touting they used factory seats, but Roadtrek covers, because they were totally tested safe in OEM testing, but of course sales guys often have no idea what they are talking about.
Manufacturers were always very proud to say they used Flexsteel seats but no literature says that for Roadtrek front seats in the brochures.
Where did you find out this, as I am very curious. I put a Chevy power track on our driver's seat and the frame had all the correct holes in place for the oddball front mounted controls for the seat, and Roadtrek never offered that option. Folks have also replaced the seat bottom foam with standard Express van seat foam and it fit perfectly also and the seat shape is identical to the stock vans.
For the OP, there does appear to be one place now making replacement covers, though, in both tan and grey.
You can get full cover sets in grey cloth but only bottoms in tan, it appears. Ours are tan, but grey cloth would look just fine with the black dash and dark carpet we added. The cloth would also not be so terribly hot and unbreathable like the stock seats, which can get very uncomfortable when you stick to it so bad it nearly pulls your skin off. I never even consider driving a long distance in shorts for that reason.