We have fixed the problem!
First, as suggested above, we adjusted the ignitor gap, and that did not make any difference. We also tested the gas pressure, and it was at least 11" WC, so that eliminated that as the problem.
As I mentioned at the beginning of the thread, upon consulting with a Suburban engineer last year, he informed us that the only difference between the NT-12SE and NT-16SE is that the latter has a higher BTU burner. So we took a close look at the parts lists for both models, and the only parts that were different were the gas orifice and the fan. That made sense -- you let more gas in, giving you more fire, and you have a higher-RPM fan to blow the increased amount of heat.
Since our NT-12SE was already disassembled, we looked at the sticker on the fan in it, and its part number matched up with that of the NT-16SE -- so that may actually have been causing the ignition problem by blowing too much air over the ignition source. In any event, now the only thing lacking was the new orifice, so we bought it (a $10 part, delivered). Its size was indistinguishable from the NT-12SE orifice to the naked eye, but we know it was larger due to the marking on it on the outer edge. We installed it, and walla, the furnace fired right up the first time. And the second time. And the third, fourth, fifth, etc. Viola! Now we have an NT-16SE. We reassembled everything, reinstalled it in the trailer, tested for leaks, and it works fantastic. We ran it for a bit and monitored the furnace cabinet to make sure there wasn't any excess heat being generated, and we didn't find any excessively warm surfaces.
And furthermore, there is no more gas leak, so presumably the old valve was leaking somewhere.
Thanks everyone for their feedback!
Dave
