Usually a bad connection somewhere between the spark generator at the twist knob and the sparker location (either a central location where a pilot would otherwise be, or spark gap at each burner). Could be open circuits (broken wires or connections) or shorts from insulation failures in the wiring.
Mine was the high voltage wire to the central sparker unplugged spade connector at the spark generator. As RVs rattle down the road a lot of things shake loose.
Failure of the ceramic spark generator is the least likely mode. However, a RV tech might be quick to sell you a new one.
If the spark gap location is central (i.e. in a pilot position) and only one burner fails to light, it could be a blocked communication tube (spiders love the scent of the stinky stuff added to LPG). Dismounting and clearing out these tubes might be considered routine maintenance for RVs spending most of their time in storage.
In any case, with a central spark location, as with a pilot, some patience is needed. The burner needs to be turned on long enough for some gas to make its way up the tube from burner to sparker, to be ignited there and flash back to light what is coming out of the burner.
Fallback for any gas burner is a flame held close to the burner as gas is turned one. I like kitchen matches (used to light gas stoves since the early 19th century) but will put up with safety matches or butane lighters.
Tom Test
Itasca Spirit 29B
