Never looked at that type but they all do pretty much the same and that one is about as basic as they come, so maybe some ideas to start you.
There are two things that I have found several times. One is that it is easy to stop gas flow in weird ways.
Under that black center plate, there is likely to be a main center point where gas comes in an then a larger circle of some sort of holes which spreads the gas and flame all around the circle.
In home use, that black circle just lays there but it may be screwed on for rV, but pull it off and look for where the gas first comes in, likely near the center before spreading out.
I something like pancake batter gets on that hoe and dries, it can actually stop the gas, so may make that a first easy step to look for trouble?
To go further, check the knob to make sure it is actually turning the metal shaft it sets on! They can crack or ream out and you turn the knob but not move the shaft to open the valve!
If you need to get deeper, it looks like a set of screws hold the metal plate to the base. Pull all the knobs and burner part to see if it lifts off without taking any of the valves loose. I normally expect the gas valves and lines to be connected to the base but the top usually comes off as a separate part.
Inside expect to find a larger gas line going to some form of setup to feed two smaller lines to the burners.
Turn the gas off at the tank for safety, open line going to the failed burner and then slowly open the gas valve to see if you have flow coming out to the line you have removed.
Fire on the small says gas is getting to where they split, so there are only a few things to stop flow to the big burner from the point where they split.
Careful! Not to let a bunch of gas run out into the RV and then light it!
We expect to smell it but if old, the smell may not be there in the gas, so don't assume it safe because you can't smell it!
Try to get lucky and find the knob is broken??? Sometimes the shaft gets stiff enough to need pliers to turn it and that can break the knob!
