My Ram 5500 is just over 10,000 lbs when empty and the Ford F550 will be close to that. That leaves you with about 9000 lbs but realistically around 8000 lbs to work with. Will you be towing anything behind you? I have an enclosed trailer that adds almost 1000 lbs of tongue weight.
Once you start looking at other trucks, you will find two problems:
1. Cab height is taller so your cabover will have to be shorter or aft of the cab
2. Wheels are taller so your camper floor sits higher
At some point, the camper starts to get too big remove. You can make modules, like JFet, or decide to hard mount it to the truck. Hard mounting takes away some functionality of being able to split the two units but will reduce weight since you do not need a loading platform and a supporting camper floor.