I'm not that familiar with RV Furnaces, but when my home furnace does that (a Comfortmaker unit), it throws a 3-4 (34) code, which means to check the Flame Sensor unit. On the home furnace, it's a single electrode which protrudes into the flame chamber. It'll fire up, burn for a few seconds, then shut down, since it thinks it's feeding gas with no flame...
One screw, pull it out, clean off the oxidation with some emery paper, rough up the place where it screws to the furnace, reinstall the screw. Usually resolves the problem. Since your unit is new, I'd wonder if it's a defective sensor, bad ground, or a bad wire. Was it working before?