Automotive alternators are middling in efficiency, IF you drive it at the correct rpm, AND if you get the belt and pulley setup just right.
Likewise, diesel engines can be quite a bit more efficient than gasburners, if you give them the right sized load. Most diesels are at their most efficient when loaded to about 70-80% of their total capability.
Here's a nice article that summarizes that idea:
https://www.ckpower.com/keys-to-running-your-generator-efficiently/
How many kilowatthours per day are we talking? Then we can do some basic sizing of components and so on.
Once we know what your daily needs are, and how many hours you want it to run, we can optimize the size of the prime mover and the generator head, whatever that is.
What kind of diesel engine are you contemplating?