Rambler, we have 5500 Onan in our Trilogy. I can't tell you about fuel consumption, but it's so convenient when you need to run both AC's, for everything else we have solar.
If I had to do it again I would probably install a heavy duty tool tray in the generator compartment and mount a Yamaha on a slide out tray, when you're parked and run out of fuel it's easier to find gas than propane, esp. with the underbelly tank, but that tank should last a long time. The generator tray would have to be pulled out of the compartment for proper ventilation of course.
2013 RAM 3500 Dually Longhorn
2014 Trilogy 3650RL, W/D, Onan 5500, 8000# Axles, IS, 960 Watt Solar, 6x6 Volt Battery Bank, Freedom SW 3012 Inv/Charger