Hi Phil,
You can not run cheap heat system and the furnace at the same time. It is either/or not both at once.
As it comes from the maker, it will not switch over to the propane furnace if the power goes off. They do not offer an add on kit to make this possible. One has to design a circuit and add a relay to do so.
They do have a model that is wired to work with 30 amp RV's. But it can not be used at all if the shore power is only 15 amps.
As Harold points out the 30 amp version can be wired to use a 50 amp pedestal. However they don't offer a kit to do so.
The 30 amp version will not meet the heating needs of the climate where I live. It is a bit better than a heat strip, because it offers more wattage and the heat does come from the regular heat vents.
In "the only game in town" rv park I am in, they would charge me extra for a 50 amp and a 30 amp connection to their pedestals. In fact they even charge extra if you plug into 30 amp and 20 amp at the same time. Fortunately I have a 50 amp "break out box" that allows me to use both legs of the 50 amp service on my modified RV.
In short, a 30 amp rv may be better off with a space heater, and it is certainly cheaper to do so than to modify the furnace by adding the "cheap heat" system.