Furnace issues

Ok, thanks for all the feedback. I will try opening up the thermostat and seeing if I can jumper the 2 wires to test out the thermostat.

The Fleetwood Santa Fe is a popup camper. Having owned a similar unit I'm quite sure the thermostat does not control the A/C. They had the usual Atwood mechanical thermostat that have been used virtually forever. As stated above, touch the two wires together and see if the furnace works. If it does, trash the thermostat, and go buy a non-programmable digital one at one of the big box stores. They work much better without the big temperature swings of the stock thermostat. The digital won't fit in the mounting hole of the stock one but a little bit of ingenuity can overcome that.

If it's an Atwood furnace, short the two blue wires to bypass the thermostat.

On some systems, the same thermostat is used for the AC and the furnace, and often the air conditioner control board also controls the furnace (by connecting and disconnecting the control wires that lead there). It's possible if you have such a system that there's a problem with the air conditioner control board...just another thing to be aware of and maybe check.

Don't change out thermostat, yet!
Remove the cover, look to see if has two wires. If it is a two wire thermostat, connected the two wires together. Fan should start immediately followed in 20 seconds or so by the burner. Firnace should produce heat. Then separate the two wires, the burner should shut down and after cool down period it should shut off.
If this happens the thermostat is bad. I would suggest as a replacement a battery powered electronic. I have a Lux brand.

Pretty sure there is no click. When I open it up on the weekend I will verify and maybe try changing out thermostat. Quite odd that it varies so much on start up time.

DC from thermostat has to 'trigger' furnace DC to FAN.
No Fan..nothing else can happen in Furnace DC circuits

Do you hear a 'click' at thermostat WHEN you set temp you want?
No click......thermostat probably issue
Yes click....then either timed delay relay or circuit board blower circuit is issue (depends on which furnace you have)

I will have to look at my manuals when I have a chance to open the trailer up again, but no the furnace does absolutely nothing until it decides to randomly light off.

Make and model of furnace?
Make and model of thermostat?
Intermittent could be the thermostat
Does furnace make any noise when it does not start? Fan come on?