This all is why I like having a Yamaha EF2000is (CARB compliant for those that think about those things) snuggled into a custom box on the frame rail. It can run right in the box, comfortably. Just remove a couple of cutouts on the box and it is vented great. Keeps it nice and secure and protected from weather. That generator can even be had in a tri-fuel version.... propane, NG, and gasoline. You can use all 3 fuels in one generator and select the fuel you are using at any time. Gas one day, propane the next, NG off of your line at your house if you have NG. One could strap a propane tank to the catwalk behind the sleeper if they had a mind to do so and run the generator.
I use it for shore power for my Xantrex inverter / charger. Inverter runs off batteries, but when hooked up to gen or shore power, it also provides 50 amps to the batteries to keep them fully charged up and ready to go. I have a Wolverine internal oil pan heater on the engine. It gets plugged into the generator on those really cold nights. The oil stays up at around 150F and flows real nice on the coldest mornings, and the heat rising from the pan keeps the engine in decent starting condition. Not quite as warm as running the block heater, but adequate. And it only uses 500w compared to the block heater. I could power the block heater if I chose.
The generator can power the inverter/charger and the oil pan heater together and still run at low speed and very quiet. About 1/4 of the generator rated load. And do it while only using about a gallon of gas in 8-10 hrs. I operate mostly in the upper midwest, so Air Cond in the summer is not as critical as down south. But if I need to, a small window AC unit can do the trick just fine. The Columbia I have has a deep groove that runs around the cab door windows. A custom cut 3/4 piece of plywood will fit right in there, with a cutout for a window AC. A Webasto heater provides in cab/sleeper heat in the winter.
Nice thing is, any of the components, Xantres inverter/charger, generator, or AC can be easily replaced with another. An authorized dealer and repair for the generator is 11.5 miles from my house. I can leave the generator with them for repair and still continue my week. And when I need it, I can yank the generator (all of 50 lb) out of the box and use it for camping or power around my property away from the house.
Total cost is not more than 1/5 of a full blown APU, and at most, total weight of everything might be 150 lb for box on frame rail, generator, inverter/charger, and window AC. Without the window AC unit, which I rarely carry with me, the total weight of everything else is less than 100 lb.
And best of all... I have all the under bunk space still available to me and not choked up with APU supported stuff. I use Rotopack gas containers that are fully DOT approved and can be thrown around, dropped on the ground, etc and not leak a drop. They are serious heavy duty gas containers. A couple of them ride in another tool box on the other side. They have cutout and could be actually mounted to a frame rail with lockable mounts if one didn't have a tool box to throw them in. And they have control flow nozzles so very little risk of spill when fueling. They are designed for serious 4 wheeler ATV off road use where they will get smashed around, dropped, fall off the 4 wheeler and skid across the ground, etc.