Dealer may be correct on this one. If the door is constructed like sidewalls are, laminated and pressed together under vacuum, glued together, it might not be as simple as replacing the interior skin. Might be near impossible to remove without tearing the entire door to pieces. I would contact Lippert, the door manufacturer, at 574-537-8900 and see what they can offer. They might not be able to provide anything except the entire door. Which would need to be the correct color of gel coat and the required decals or stripes installed. Give them a call. Have the information from your door tag handy as you will need it. Date of manufacture, model, size etc..
Chris & Maria
2012 36RL, Full Body Paint, Dual Awning, Dual a/c, Splendide Combo, Fisher Paykel dishwasher, 6.5kW Onan, Goodyear G614's. 2015 F550 6.7l, 4X4, Lariat, CC, 176" WB, Long Box, B&W Companion