I would inspect the truck and trailer before leaving, and do any PM that might come due while I was gone.
I would leave 1 generator at home. You're leaving in September, which can be a very hot month once you get to AZ,NV,CA. Unless you think you will be that far by October, you won't need air conditioning. Other than A/C, why would you need 2 gen?
I don't carry spares of anything. I long ago stopped playing that game where you try to guess what is going to fail, and then it is always something else that fails. Then next time you buy that thing, and it will be something completely different that fails. Soon you are carrying way too much stuff and suffering stress related heart failure. I make exceptions if something has a pattern of failing often, but only if failure of that item would be a major issue that I couldn't live without.