This is just a hunch, but you may have a problem either with a connection/wire going to/from the sail switch or a problem within the sail switch...
The way the furnace "should work" is that when the thermostat calls for heat, the control board FIRST looks for an open in the sail switch, then after the fan starts running, the control board looks for the sail switch to close. If that sequence is not followed, a faulty sail switch "COULD" create an unsafe situation. So the control board "verifies" the sail switch is operational as a part of every startup sequence...
Then, if the sail switch "flickers" or "opens momentarily" through the furnace heat cycle, the control board "senses that change in continuity" and shuts down the furnace as a precaution....
In other words, the control board "monitors the operation of the sail switch" throughout the heat cycle and if there is any indication of an open/short in the sail switch circuit that is not expected or that does not follow the exact sequence, the control board shuts down the furnace.
The way the two quotes from you above read, is it possible that your sail switch is not "following the expected "open, closed, remains closed and then open" sequence ??? Remember, the continuity from the control board, to the sail switch and back to the control board includes not only the actual sail switch, but the connectors and wires that attach the sail switch to the control board. Also, to complicate things, the printed circuit/components on the control board may be at fault as well as a "spike/drop in voltage" from the trailer to the control board that allows the fan to drop below a specific speed that would keep the sail switch closed....
My "hunch" (and that's all it is, is a hunch) would be to focus on the sail switch and make sure that its ENTIRE sequence of events follows the protocol. There may be nothing out of sequence, but if there is, that could be the source of your unexpected shutdowns...