I got a 5er for several different reasons.
1. I have a diesel pickup. I think it is way more costly to get a diesel motorhome then to get a diesel PU and a 5er.
2. I wanted the drivetrain separate from my 'house' for maintenance reasons. If something in the drivetrain breaks, I can still use my house. Put the 5er in a CG till the truck is fixed. All I need is a rental car to get around while I wait. Probably not so easy if you have a MH.
3. There are 50 million Ford dealers in this country and any of them can work on my truck. I believe you will need specialized service centers for a MH. And there will be considerably less of them. Oil changes and other maintenance is easy - I just run into town to the local dealer.
4. If I choose to buy a new 5er in 5 years, it will be much cheaper, since I already have the truck.
Ultimately it comes down to what you like better.
Tim