It says parking is available, which means parking on the main land (at 12.50 Euros/day) and taking a shuttle closer to the Mont then walking to the hotel followed by climbing the stairs with your luggage. All of this was fine for us, we're in good shape, but I can easily see how this would be a burden for some. We left the 'Do Not Disturb' sign on our door when we went out, because our room contained valuables and we would be back in an hour or so. When we returned the sign was removed, the door was unlocked, and they had made the room anyway. I informed the manager who seemed somewhat concerned that we were frustrated but still didn't understand why it would be an issue.
Find yourself a cheaper place to stay and do take the free shuttle to experience the truly incredible place that is Mont Saint Michel.
Guess...the location. Although, this is overstated in some ways. Staying on the actual Mont was nice but it really wasn't worth the enormous cost, in our opinion.