any gasoline engine that uses a carburetor is easily converted to lng or lpg.
many different kits are available. not uncommon.

You can get a pretty much bolt on conversion for a any common gas or older diesel truck. Diesels may be even slower, but you can install an exhaust brake for better safety and less wear.
A half ton truck will do it, but a 3/4 or 1 ton will do it better.
A stickshift will last longer and give a little more control, but an auto is nice in traffic, and less to think about while maneuvering.
A lifted 4x4 handles worse, but 4x4 can help. A dually is more stable, but they suck in tight spaces.

How long will lyou drive at a time? How often?
Are you in the mountains?
What kind of budget do you have?.
Is driver comfort or appearance a factor?
How much non tow use will the vehicle see?
Single or double axle trailer? Does it have brakes?
All these are important factors in your choice.


A Chevy Ford Dodge 3/4 ton with 4.10 or steeper gears, 1972 or newer will do the job fine, but a 78 will obviously be less comfortable than an '08, or even a '98. A small block V8 will do it, but may struggle on hills. A big block will burn some extra fuel.
I'm not trying to bash imports here, I'm sure a new Tundra on propane would be a great truck, but its beyond most of out extra vehicle budgets.
My experience is that any towed load that weighs less than the towing truck isn't too bad, but you have to be much more careful as the load starts to outweigh the tow rig.