I have the same year and model as you. I measured my mattress today and it is 70" wide. I measured the bed made up with the duvet on top and it measures 72" I also measured the space inside the slides and it is 74" so you can put a 72" mattress in and it will fit. I have that same crappy mattress that came with the RV, jbut we put a 3" Ca King mattress topper on it (from Costco) It was too long but I shortened it using my electric knife (what they use at Joann to cut foam). We have only slept two weeks on it so far and it was very comfortable, but we know it will eventually break down and we will replace it with an RV king air bed and use the foam topper. It really irks me that they put such cheap mattresses in an expensive rig that is geared to full-time use. We will be living in it full-time in 10 days so we'll see how long it lasts.
As to linens, I used king sheets and got a pair of mattress suspenders to take up the extra fabric on the foot end of the fitted sheet. It works well and the sheet doesn't bunch up after sleeping on it. We use a comforter with a duvet cover and forego a top sheet. I wash the duvet cover about once a month. It makes making the bed a snap. We even have two different down comforter inserts; light for summer and warmer for winter.