I had very similar problems with a relatively new Suburban NT-30SP furnace. The problem arose suddenly where the furnace would go into hard lockout mode after about 6-10 minutes of running. With the front cover removed, the unit would run indefinitely (until the coach reached the thermostat's set temp). For this reason, I assumed the high limit switch was putting the unit into lockout. However, this was not the case - it was one of the two solenoids on the gas valve that was failing intermittently - and only when it got warm. The loss of gas put out the flame, which couldn't be relit and so the unit would go into lockout mode.
That was the short answer, but I will provide details below for anyone experiencing similar problems:
The furnace is a Suburban NT-30SP, bought new in 2017 and installed in a shell-off '75 Airstream Argosy renovation. Spent a full winter living in the trailer (lows around 0 degrees F) with no issues. Then sat winterized for two winters. Got it back up and running for about 2 months in the winter (again, lows around 0 degrees F) with no issues. Winterized it, it sat for 2 weeks, then tried to boot it back up in December and the furnace would not run for more than 10 minutes before going into a hard lockout. Again: with the front cover removed, the furnace would continue running fine and not enter lockout mode.
Tried heating the coach with electric heaters before starting it up again - didn't make a difference.
Cleaned and then tested the limit switch in my oven with a meat thermometer and multi-meter - it did not lose continuity up to 215 degrees F, so I assumed the high limit switch itself wasn't malfunctioning.
In case a mouse got in somewhere or something and airflow was being obstructed, I took apart the entire furnace and vacuumed it, blew compressed air through the manifold per manual's instructions. Did the same to the 4 ducts. Re-measured cross sectional area to verify I was within spec on that. The unit was fairly new, and so was really quite clean all around. There were no obstructions in the ducts either. Reassembled, and the problem persisted. (Also, the intake screen for this unit is huge - about 24" wide by 16" tall, and anyway, all these tests were done with that screen removed.)
The water heater had been making some loud noises, and I wanted an auto-changeover regulator with a color indicator (which the existing one lacked), so I bought a new regulator for the trailer without testing any pressures. The problem persisted.
Got a digital manometer from a friend and checked the coach's propane system's pressure - 14.15 inches of H2O. That's a tiny bit higher than the furnace's stated range of 11" to 14" H2O. Without adjusting that regulator, I tested the pressure coming out of the orifice downstream of the gas valve - 12.05" H2O. That's quite a bit higher than the 10" to 10.5" quoted by someone in one of these forums as being from the manufacturer. The gas valve does indeed have its own regulator built into it, and printed on the side of the gas valve are the words Reg 10.2", which I assumed meant it was supposed to further regulate the gas pressure down to 10.2" H2O - though now I'm not so sure. Anyway, I bought a new gas valve, which comes with the 2 solenoids already attached. Unfortunately, the package got crushed in the mail and both solenoids looked broken, so I swapped them out for my original ones on the gas valve I was replacing. Reassembled, and measured gas pressure coming out of the orifice - even with the new gas valve, it was still at 11.46" H2O. Reassembled the burner tube, and some new data: this time the furnace had trouble lighting, but on the 9th try (3rd attempt of the 3rd cycle), it finally lit, but unfortunately went into lockout about 2 minutes after I replaced the front cover.
I adjusted the trailer's regulator, counter-clockwise 2.5 full rotations (a little at a time while measuring) until I got the system's pressure down to 13.2" H2O on the main tank and 12.7" H2O on the auxiliary tank (those auto-changeover regulators lose a little pressure when they changeover). With the water heater running there was a 0.2" H2O drop. Hooked the gas back up and tried again - this time the furnace refused to light and I gave up after 9 tries (3 cycles of 3 attempts).
I removed the gas valve and solenoids and bench tested them off my car battery. One of the two would not click with 12.5V applied, and I could not blow through the valve with 12.5V connected to both. Of the two crushed ones that arrived in the mail attached to the new gas valve, it turns out that one still worked and the other did not. So I used the 2 good ones I had - one from the old gas valve and one from the new gas valve. Applying 12.5V to both opened the valve and I could blow through it. The polarity on the new one was backwards from what was stated on the bracket that holds them in place, so I amended with a sharpie.
I went back and reassembled and tested pressures again - for science. Coming out of the orifice, pressure decreased with every iteration until plateauing at 10.75" H2O. (First cycle was 11.2, 11.05, 10.85, second cycle was 10.82, 10.75, 10.75" H2O). Reassembled with the burner tube and everything worked and the unit us no longer going into lockout.
I haven't looked into it (though I'd like to), but maybe solenoid's start to fail with higher heat? - not heat so high that the high limit switch thinks it's dangerous, but some level of heat between cold and normal operating temperature?
To recap - it was a solenoid on the gas valve failing intermittently and not an overheating situation caused by some malfunctioning of air flow or gas flow.
Hope this helps some people out there. Cheers.