fifth wheel furnace

One thing you might check: there is a metal sensor (don't know what it's called) that is supposed to be kept hot by the flame; the gas is cut off if the sensor doesn't get hot. Sensor can become coated with carbon buildup. (That happened to my home furnace.) You can clean it with fine sandpaper.

Could be sveral things
Low batt voltage
Stuck sail switch
Dirty or bad thermal couple
Dirty orfice
Bad board
Blocked chamber
As suggested on hundreds of threads on here that is a start for you to check

To the OP: Why are you SHOUTING AT US?

Where is your thermostat in relation to the closest heating vent. My thermostat is just outside of the living room/ bathroom door and the closest floor vent is just inside the bathroom, so if I leave the bathroom door open the air blows directly on the thermostat and causes it to short cycle just as you describe. Just something to consider before you begin throwing money at the "problem". Could be as simple as closing a door or redirecting a floor vent.
Good luck / Skip

Warm air blowing on t-stat will cause cycling problems but usually longer duration between cycles.

The short cycling you are experiencing could be due to:
Restricted discharge air flow.......make sure all registers are clear and flowing air
Restricted return air flow.....make sure that return side of furnace is not obstructed
Furnace over-firing due to High propane pressure....pressure should be 11" WC with at least 50% of other propane appliances in service.
High limit switch is faulty.......190*F switch (need to pull furnace to access)
(You can check for 12V fluctuation at connector on circuit board----white wire marked AIR on board. If limit switch is opening/closing power will come/go at that contact)

RoyF wrote:
One thing you might check: there is a metal sensor (don't know what it's called) that is supposed to be kept hot by the flame; the gas is cut off if the sensor doesn't get hot. Sensor can become coated with carbon buildup. (That happened to my home furnace.) You can clean it with fine sandpaper.

That would be the thermocouple right?
It needs to be in the path of the flame, but in the correct spot.

Do you have a good flame at stove top? If not you may have turned on the propane valve to quickly. You'll need to turn off the valve, wait a few minutes and then turn it back on slowly.