Heat stains in vinyl ??

For the last couple years of Summer storage, the vinyl floor in front of the ramp of the toy hauler has been developing a dark rose colored stain. it extends about 36" from the door, full width of the trailer...I thought it was caused by the exhaust of the Prowler when I would run it every couple months without dropping the ramp. I didn't start it this Summer and the stain is worse than ever..There is no stain under a piece of masking tape I put on the floor as a guide to load the Prowler. It won't wash off, bleach injected under the top film does nothing. I open the ramp every day to the sun and in a couple weeks the "stain" is almost gone................
Last year I bought a heavy cover for the boat. When I opened it up to get ready for cooler weather boating I found the vinyl seat back was "burned" where the cover was touching. I figured a re-upholster job but after several days in the sun the "burn" is gone. I had the back of the captain's chair covered with several layers of a packing pad and it's "burned". I uncovered it today to the sun hoping for a cure.
What's going on here???? :h Thanks to the genius with the answer.. :)