I have a built-on Onan and always carry a portable generator when boondocking, because:
- It makes less noise than the Onan (an eu2000i), and
- It uses less fuel.
Yes it's one more thing to maintain, but I occasionally use it at home when it's easier than running an extension cord, or when the power is out. I've never carried extra fuel because a full tank on the generator has always met my needs, and if I did run out I could use the Onan until getting fuel. Yes, it does take up more room, and that's the biggest drawback for me. For me, the portable generator is primary and the built-in is the backup.
It's really a question of your needs and preferences. Running a "big" generator to top off the battery seems very wasteful to me. To somebody else it might make perfect sense.
