I also have a Suburban furnace that is short cycling after about 2 minutes, it's a 40k btu IIRC. During all the testing below the 12v system was fully charged and the fan ran smoothly.
We spent two days at a repair shop trying to determine why. First the high limit switch was checked to be sure it was the correct "color code" for our furnace -it was. Green dot I think.
The high limit switch was checked and was determined to be the component that was actually shutting the furnace off.
The switch was replaced with a new one (twice) -still short cycled.
The outside cover as well as the covers inside (the one with the duct work attached as well as the return air vent) were removed. At this point there were no restrictions on the intake nor exhaust air. It still short cycled.
The tech hooked up their propane bottle/regulator and 12v battery, bypassed the thermostat installed in the rv and it still short cycled.
The furnace and the metal "shroud" it's installed in was removed and "bench" tested as a unit by the tech. It did not short cycle.
The fire box did not have any rust/dirt/rat nests or damage, and was a uniform dark color. It was hard to see any difference between mine and a new one.
The tech called Suburban and advised them of the problem and what testing had been done. Suburban asked that the metal tab the limit switch is mounted to be bent away from the fire box somewhat and (if not already) move one of the heat duct exits from the furnace so that the limit switch was in the direct path of the exiting heated air.
None of this helped and the tech said Suburban "said" it was safe to run the furnace in this condition -just expect to replace the high limit switch frequently.
I'm not happy with this situation but am at a loss as how to cure it. I watched all the testing as it was done, from start to finish and the tech was very thorough.
Sorry to be so long winded but hope this might help in testing for this problem.
Hope you find yours geezer...
2008 Gulf Stream 8327 Fred, 300hp Cummins, Allison MH1000, Freightliner, 24k lbs chassis
2004 Ford Ranger 4x4 5spd, 4k lbs, flat towed