If a trailer has a heat strip and AC, how hard and expensive is it to put a forced air heater in it?

I found a trailer I like but it has a heat strip which I've already ruled out I do not want.

The heat strip is part of the A/C unit.  And you are correct that it's not adequet for our temps here in Northern Utah.

The forced air furnace will be a totally different unit. They are usually located under the coach  or other area near the floor. they come as a single duct that will heat a smaller LQ or ducted units that push air to various outlets in larger LQs.

The point is No parts will be common between the two.  Heat Strip in the ceiling as part of the AC, Most forced air furnaces near the floor. So just figure the cost as being a new install.

If you are getting a 6-8foot LQ, you can probably get by with a single outlet for the heat.  So it's just a matter of finding a location under the sofa, installing the unit. ( which requires some cutting of the outside wall of the trailer) routing the gas line over to the furnace.

Adding a forced air furnace to an existing living quarters is possible, but not easy.  You'll have to find room (approx 12" high, 12" wide, and 24" deep) for it at floor level next to an exterior wall of the trailer.  A propane line, thermostat wire, and 12v wires to power the blower and ignition circuit will also need to be run to the same area.  The furnace burner will also have an inlet and exit pipe running directly through the exterior wall of the trailer.  Finally, the furnace can directly disharge heated air into the LQ, or can be ducted through the existing cabinetry.  A new furnace (16k btu) can be purchased through a discount rv shop for ~$550.  Labor for installation will depend on ease of doing everything above.

Many folks get by with a portable ceramic heater, but of course that requires 110V AC power from generator or shorepower.

Hope this helps. 

I suspect the reason he is asking about installing a propane furnace is that here in the west there are very few places that we camp that have any services.  Most of our camping is at the end of a Forest Service or BLM road and it's all primitive.  Even if you are a roper or barrel racer, Most fairgrounds don't offer trailer hook ups. so it's just pull into a parking lot and park. Common courteousy  and Forest Service rules don't allow you to run the generators over night. So when it's 10* at night you need a furnace.

Also Randolph where JcNhEm1 is located is probably the coldest place in Utah and often shows up as being the coldest spot in the lower 48 states. I seriously doubt he will use his trailer's A/C much. But I know he will use his furnace.