I have a 6.5 KW version of that Onan and it all depends on how the fuel line is piped in. My genset runs off one side.
Some of the higher end 5th wheels have installed larger dedicated tanks in the frames just behind the axles. I am doubtful that the Montana frames would take that stress. One could check with Lippert directly however Keystone would mor than likely nullify all or part of your warranty. If you are out of warranty and Lippert says it will support the weight I would consider it.
2015 3440RL, legacy package, IS & Disc brakes, Sailun S637's, Onan Commercial Gen, 1200 Watts Solar, Midnight Solar Classic 150, Winston 700AH/8.4kwh LiFePO4, Winegard Trav'ler & Dish Hopper3, 2011 Ram 3500 CC Laramie DRW 4x4 4.10