I have told you already, in post #2 of this thread, tune in channels 490-494 (actually in your case, you only want to look at 490-493, since 494 is for the 110, which you don't use), now, if it reads the correct voltage (refer to post #2 for a list of proper voltages) in the banner that should appear in the lower right corner (it will say something like (for channel 490): "Test 1 SAT A (101) Odd Txp 13V"), it means the LNB is fine, if instead, it says something like, "Searching for signal on: Satellite In 1", then the LNB is bad (after eliminating the coax, multiswitch (if any), and STB as the culprit of course).
As for how it/why it/what happened, it is very hard to tell, I am not a D* tech, and I have never had any direct experience with the type of RV dish you have, it could be anything, from water intrusion causing a short, to vibrations causing the coax to stretch or break, or rub against a sharp metal edge, I just can't say.