This is not uncommon in earlier Montanas. Below is a Post made long ago and re-used in recent times about the problem.
As suggested, ensure the burner area is clear of bugs and debris. Assuming it is, the problem is most likely a failed solenoid coil (usually the one closest to the water heater). Stand outside the WH with the cover removed when someone else turns on the WH propane switch. If you hear the ignitor click 3 times and then stop, then it is very likely the gas valve is not allowing propane to flow due to a bad solenoid.
A Search using keywords "water heater solenoid" will expose lots of previous discussions on diagnosing and replacement of the coil. A good post to start with is this ONE
. Here is another POST
. Follow the other links pointed to to further understand the problem.
You can replace the entire gas valve and solenoids for around $100+. Your valve is likely still functional along with one of the coils (assuming you followed the directions of testing the resistance). But the new gas valve assembly will solve the issue (and leave you with a spare valve and solenoid)
Another solution referenced was buying a replacement coil kit. They used to be available from www.americanrv
parts.com at around $15.. Unfortunately it appears Lippert bought them out and no longer offer the kit. A quick Google found a possible source *HERE* They have a couple of different part numbers, this is a 521074. so check which ones match your gas valve. As I recall the coils are all pretty much the same but caveat emptor.