This used to be a common problem in the 2005-2010 Montanas. The gas valve has two solenoid coils that would fail. Usually it was the one closest to the WH itself. but not always. The solenoids are activated with 12V to allow the propane to pass through the valve. When one doesn't work, you hear clicking, no propane smell and after 3 tries it stops.
This is the gas valve. The solenoids are the round coils with the wires.
There are a bunch of posts on this topic, but they are old, the pictures are no longer available (thanks TinyPic!!), and the links to parts are no longer good. But a Search on " gas solenoid" or variations will uncover them.
You can test the coils with an ohm meter. A good coil should have a resistance of around 40 ohms.
The coil is easy to remove and the replacement drops right in, aand tests good
The big gotcha at the time was the vendors wanted to you to buy the entire gas valve at ~ $100. But we had found replacements solenoids in a couple of backwoods places. A guy in Quartszite had a supply, then there was a few on E-Bay and the americanrvparts.com (no longer around) had them. But those are all gone today. But I did some Google work and found that they are now easy to get from numerous suppliers! Google "Suburban 521072 Solenoid Replacement Kit" and you will find them for $18-$25 for a set of two. Here is one link
https://www.amazon.com/Suburban-5210...a-605196339293