"Does anyone have any idea of why spark, gas and air don’t produce fire?? I mean, it seems simple enough -- have spark, gas, and air -- why do we not have flame? We don't know what else to try and are about to toss the whole thing and replace it, even though they are about $500. We talked with our local RV repair shop, and they don't have any suggestions."
You have spark, gas flow etc.
Then one of two things why no main flame
1) Low gas pressure (should be 11" WC with other gas appliances ON)
2) Spark electrode is NOT correctly positioned (electrode should be 1/4"-3/16" above and directly over slots on the burner with a 1/8" gap between electrode and ground)
1/2" above burner opening is too far away.
And you might want to disconnect propane line at furnace and blow it out to insure that gas line is not blocked with oil/moisture residue.
