The first thing to realize in deciding on the loads is that you need to use the actual LOADS (i.e., ratings on the devices) rather than what the size of the breakers they connect to are. Note that the propane appliances still likely need electricity to operate their control circuits, fans, etc...

There are essentially TWO ways to do a manual transfer to a portable generator.

One is you can use a transfer switch like you have been looking at. This requires the loads to be wired through to where the transfer switch is. This can be difficult in your case where the essential loads are down on a sub panel. It's possible you can bring the furnace up to the main panel and make things easier.

The other way is to use an interlock. Here you just leave the panels fairly much AS IS. When the power goes out you run around and shutoff the non-essential loads. You then start the generator. You turn off the main disconnect for your house and then turn on the generator. The interlock keeps both of those from being turned on at the same time. The generator feeds the panel through an appropriate sized breaker (for the generator and the wiring connecting it). When the power comes back, you turn off the generator feed, turn on the mains and then run around and turn off all the shedded load.

The only other priviso is that the generator feed breaker must be screwed down to the panel. Most panels have some provision to do this (might require a special part).

Below is a company that makes the interlock

Here is a link that might be useful: http://www.interlockkit.com/connectmygen1.htm