I think you're on the right track Oscar. If the dealer can't help you, call the manufacture and tell them what your wanting to do. Not sure what material the counter top is made of but they may just have a piece of scrap or a damaged one that you can get enough off to make your extension. Some manufactures have the same counter tops available in multiple models so while your camper maybe didn't have an extension, there may be a different model that offers the same counter top that does. So you may get real lucky and just be able to buy it and not have to make it.
2020 Chevrolet 2500 LTZ, 2019 Forest River Wolfpack 23Pack15, 2014 EZGO Golf Cart.