I would look at toy haulers too, whether you do bumper pull or fiver. That's going to be our next move because they offer so much more flexibility in terms of carrying toys and other cargo - such as your kayaks. There are separate garage models and there are also options where the garage is part of the main living area. In either you can generally you can get a pair of queen bunks that drop down from the ceiling for kids with a separate master up front. Just a thought anyway.
In terms of how the trailer is configured, with a fiver you get better towing and a shorter overall rig but the tradeoff is losing much of your bed space. With a toy hauler this is less of an issue, and even with a regular fifth wheel you can generally work around the limitation if you're creative. You don't lose your entire bed and you also have more storage in the camper.
Check out an Andersen Ultimate hitch if you want one that's easy to install/remove. I use the rail mount version and love the way it works. Some don't like rails in bed but I don't mind them an actually use them all the time as cargo tie downs.
Diesel vs gas? Either works at the weights you mentioned. Your profile says you live back east so you likely won't deal much with high elevations or Rockies/Sierras much if at all. I would recommend driving both and getting the one you prefer. The diesel is going to perform a lot better towing but the importance of that is up to you. Cost is a wash overall so don't get distracted by that red herring. The diesel costs more to buy, less to operate and it's worth more when you are done with it. If you look at the total ownership cost...the difference is immaterial.
