9 out 10 times this is caused by a failed solenoid. Go read this posting
http://www.montanaowners.com/forums/...eater,solenoid Read this post and the ones linked to it for instructions on diagnosing the coil problem.
For those who don't like to read a lot, here is the gist of the process to diagnose a bad coil. Remove the wires from each coil and using a volt/ohm meter measure the resistance of each coil. A normal one is around 35. If it is open (resistance is 1.00) it is bad and needs to be replaced. The solenoid coil closest to the burner tube is usually the failed one.
Replacement coils are available for a nominal sum (v. buying a new gas valve for ~ $100) from americanrvparts.com for under $20 (that is for 2 coils - you wind up with a spare).