If you look for the manufacturer and model # of your furnace on Google, you'll find an owner's manual and probably troubleshooting guide. Most RV furnaces turn the fan on first. When the air flow is strong enough, it pushes a paddle switch (called sail switch) and signals the control board to turn on the gas and ignite it. If if fails to light in 3 attempts, it shuts down. To reset, you turn the power off and then on again.
You could have air in the gas line if it's never been used, or a spider web or bug nest in the ignition area. I'd start by blowing out the air mix area around the ignitor. You can use a can of compressed air. If these ideas don't work, you'll have to get further into troubleshooting.