If you've got the room and money
a diesel in the engine
room is the way to go.
If you don't have the room or money
I wouldn't be too quick to dismiss a propane
powered suit case generator.
It really doesn't take much more propane to run a generator than gasoline. A 4.75 gallon propane tank is about the size of a 5 gallon gasoline can. If you convert a gasoline generator to propane with the typical kit, you will get less power from it.
For example if you convert a Honda 2000 to propane the continous output will drop from 1600 watts to maybe 1400 watts but it will run about the same length of time as it would on the same amount of gasoline.
A propane powered engine
tends to require less maintainence than a gasoline engine.
Little to no carbon buildup in the valves and no deposits in the carborator.
Gasoline is just messy to handle
For me the biggest reason to go propane is safety
. Propane engines produce a lot less carbon monoxide. They can still kill you but it takes them a lot longer. I also believe it's harder to spill propane and have it end up in your bilge
All that said, my Honda runs on gasoline and I've been very happy. I'm switching to a propane outboard
so now I'm thinking about switching the generator as well.