87 Terry By Fleetwood Furnace Replacement?

What would be a good furnace replacement in my 5th wheel Terry by Fleetwood? I have a diesel heater but it would be nice to have a backup just incase. My furnace works but the exhaust fan tube is rotted out ( I probably called it the wrong name) its a tube that runs from the front of the furnace to the rear, to a what looks like a small fan housing. I would like to upgrade it to something better or even having majority of the replacement parts since the main exhaust/fire tube is a bit rusty. Like I said I have a diesel heater (I have yet to mount it) but having a propane furnace as a backup would be nice 

Thank you!