Hi Gary,
Find out the btu output to the RV. Check the two speed unit and see if it is a match. I'd not trade longer run time for electricity when boondocking (unless you have solar) is a scarce resource.
I think you may be a candidate for a Platinum Cat. They are thermostatically controlled and do have a chimney (read, totally safe to use when sleeping). The power vent fan just sips electricity compared to a furnace fan, and you would have the furnace to pick up the load if there is a sudden freeze. Cost would be less than a two speed furnace. If I ever get one I'll locate it in the bedroom as there is already propane plumbed in to just below with a quick disconnect.
If your OEM furnace is shot--then by all means upgrade to a two speed unit.