If I get a truck camper, here is how I'd solve that problem. I looked at a number solutions for this:
Moving the spare to the front, installing an Onan where the spare was, teeing it from the vehicle's fuel tank. Problem is... nobody anywhere has done anything like this.
Putting a rack for a generator on the TC. Doable with a 2000 watt Honda, but not a 3000 watt Yamaha.
Adding a second alternator, or going whole hog with a MEPS system. Cool idea, but I don't want to add wear and tear to my vehicle's engine.
Having a custom front hitch receiver/bumper fabricated with a special rack for the generator.
If I go the truck/TC route, I am going to go for a front hitch receiver and a cargo carrier, then plop my 3000 watt Yamaha on that, securing it. This way, I have a generator available at all times, and it is in a secured location.
