Sounds like this could be your bus. It has the 5.9 cummins which is by far the most common of the diesel bus engines as it is also used in a few trillion dodge pickups. This makes parts/repairs more affordable. This bus has a few other things going for it. I think it is the perfect size. Not too big, not too small, especially for a single person or couple. Looks to be around 30 ft? And since you are used to living aboard a boat, you will have more room than you will know what to do with. I suspect that better than 10 mpg with this rig should be quite doable given it's size/drivetrain. That's pretty good for a bus.
The biggest thing this bus has going for it is it's windowless from the factory which means you can install them where you want and you won't have to worry about skinning over existing ones. This can be made into a very clean looking RV which doesn't scream out "I USED TO BE A SCHOOL BUS!!!!!".
Another plus is, it's right down the road from lovely nawfuk. I lived there for 4+ years while in the Navy.
Get out of there before winter sets in. It is cold wet and miserable. But, then again, being a limey, you're probably used to that!
Well, good luck, mate. Having spent time on boats, I assume you are familiar with their systems. If so, you should have no trouble working with RV systems which are quite similar.
And if you do get it, fix it up nice and come visit me in Connecticut. I'll buy you a pint! I gotta show the wife a well done skoolie. Maybe she'll let me get one, then.
As for towing a dingy, no problem. You could drag a small car, or a bigger one, for that matter and not even know it was there.