My adult son uses a wheelchair and we have the same problem. Today, however, I made a reservation through the Marriott "ADA Department." A very nice lady named Terry helped me find a Marriott property in our destination city (Christmas visit to Grandmother) that had a roll-in shower and she got it reserved for us in just a couple minutes. She contacted the hotel itself to confirm the availability. It was quite easy for me to make this reservation. Terry understood the issues, as she has a person in her own family who works with disabled folks.
So, if you are having trouble finding an accessible room call the ADA Department, which is reachable through the Marriott reservations number, 1-800-321-2211.
We recently stayed at Ocean Pointe in Palm Beach Shores (Singer Island, FL) and were very pleased with the accessibility features of our 2 bedroom condominium (timeshare). The master bathroom had a roll-in shower and access to the main patio was very easy because there was no threshold. Staff immediately brought us any additional equipment that we requested. We were truly amazed and so thankful when we found we could borrow a specialized "beach" wheelchair with extra large wheels for the sand. The whole experience was more than we ever dreamed for and we can't wait to go back next year!