Check 12 volt DC wire connections to the furnace. May be loose or corroded and ignition circuit not getting enough power. When you find the circuit board, unplug it and clean the contacts as suggested above. Troubleshooting furnaces can be tricky, a number of safety features that can cause problems. There are jets that can clog, ignition system parts that fail, sail switch, etc.
If you lack confidence and need to get it working quickly and reliably, a good mobile RV service may charge you some $200+ to diagnose and fix it. Watch repairman if posssible and get familiar with the innards. Maybe there is a You Tube video about this.