There are a couple of issues. Backing a boat in with a MH can be difficult because of the length and width. If you are not leaving the boat in the water, you'll have to break camp every time you want to launch the boat.
There are more rigs available today that have larger beds, so that will help. You are looking at a Class C or Class B MH, or even a truck camper, depending on how much interior room you want. It's a good time of year to check out a Boat/Outdoor show to see the inside of a bunch of models.