Hi Macman43, check out this posting from Beachnut:
https://www.gsowners.com/forums/showthread.php?t=6332
They had a custom mattress created for the 5245 that was 2 sections with a fabric "hinge" on one side so the bottom of the mattress would slide up the bathroom wall as the slide out retracted. I am still using the original wedge mattresses but looking for improvements as it still does not work smoothly.
Good luck and I will look forward to your innovations!