Furnace Won't Ignite

Symptom: Blower runs but the furnace doesn't light, when it is turned on at the thermostat. 
Furnace: Suburban nt20seq
Background: The furnace has caused me trouble (could be operator problems) since I picked up the trailer.  On the first night, I could hear it spark but it wouldn't light.  I cleaned the outside vents and it lit fine.  Then it wouldn't light again.  I decided to put off dealing with it until I came home.  When I got home it lit, but I'm not sure why.  Now it won't light again.  What is different now than from before is that I don't hear it sparking. 
From my reading I find 3 possibilities: 1. lockout; 2. sail valve; 3. controller board.  I don't know if there are more possibilities. 
Of these 3, I think the only one I am competent to "fix" is the lockout problem.

Answers to the "have you done something stupid questions" are 1. the gas is on, as has been checked by lighting the burners on the stove; 2. the pod is plugged into 20 amp power; 3. the fuse for the furnace is OK.

First Attempted Lockout Fix:
The information for resetting the furnace from a lockout is pretty thin.  The two pieces of advice I found are to recycle the furnace at the controller a few times.  What I believe this means is to turn the furnace on and let it try to light.  After a minute or so turn it off, wait for the blower to stop, and then try it again.  I went through this 4 times and have concluded that this isn't going to fix my problem.  I am not sure whether, or not, this approach clears a lockout.
Second Attempted Lockout Fix:  I lost this reference (this is at least one stupid thing) but as I recall it was to turn off the gas; cycle the power to the furnace; turn the gas on; try to light the furnace.  As I said, I lost the link to this piece of advice.  Anyway, I turned off the gas and then pulled the fuse to the furnace to be sure it had no power.  I lit the burner on the stove and waited for it to go nearly out to be sure the gas was off.  I then waited for 10 minutes and turned the gas back on.  I then put the fuse back into the furnace and tried to light it.  That failed.

Is there any more I can reasonably do or is it time to seek professional (not mental, but service) help?

elko mike, I had a similar problem, was ready to start opening gas lines to see if gas was getting to the heater, (the last gas appliance on the circuit) I took the tank in and made sure it was filled, same problem, furnace would not light. Finally with a little help from this forum, I found out my 20 lb cylinder has a safety valve that will not let gas come out to fast. I had to shut off the valve to the tank, wait for a few hours, slowly and very slowly open the valve, wait overnight, made sure cook top would light and stay burning for more than 30 seconds, then the furnace lit and worked every time. I hope that is all the problem you have, like you I could hear mine purge blow, then spark then putt, then out. after leaving the valve on for 24 hours it was fine. good luck.

Thanks folks. 
It lit this afternoon, which adds to my confusion.  I let it sit with the LP on and on a lark tried to light it this afternoon.  I could hear the ignition system working.  I did have to cycle the controls a few times, but it did finally light.    In that regards, Dane's post is similar to my experiences. 
One post I found on the net mentioned the need to purge the system, although it wasn't too explicit as to what that meant.  I took it to mean replace the air in the lines with LP. 
The idea of opening the valve slowly is interesting, as it seems some deliberate set of actions should result in the furnace lighting every time. 

I just bought a slightly used 2011, 177 and tonight I tried the refrigerator on gas for the first time. The ignition clicked repeatedly, then a tiny puff of ignition, but never did operate. No gas?? Is there a shut off valve somewhere for this appliance. The hot water tank worked on the LP when I tried that last, before taking it to a dealership for a different plumbing problem.

Sorry everyone. I have no owners manual, the former owner knew nothing about the trailer, and of course I had no dealer start-up instructions.

On our 177 I do the same thing: light the burners then the fridge will light.  Then the water heater, nd last the furnace.