I would say the RPG is moderately supported.
while that may not sound real promising its about as good as it has ever gotten.
MechWarrior 2nd edition did have more support, including some adventures/scenario's but my understanding is they never really sold well enough to really justify them.
with that said ATOW does have conversion rules so you can take characters/stat blocks from the older editions and use them in ATOW, you would also need to replace weapons and armor with the current edition versions, and similar.
with that said the RPG has a little background but its not focused on that.