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It most likely will not be a bad flywheel.
A bad starter is a definite possibility.
But from what you have described, I would also have to wonder if you have internal engine damage - bad bearings.
You need to pull the set out of the coach and get the starter off.
Inspect and test the starter.
Yes, it very well could have.
If you are noticing a drag when manually turning the engine, it is either going to be an internal engine issue, or something hung in the drive system.
This is a belt driven set, and it is possible that something has become loose underneath.
Unfortunately, the only way to know for sure is to tear it down.
On the drag, though, are you sure it is actually a drag, and not just hitting the compression stroke?
Both of those are very 'iffy' questions.
Honestly, if it were my set, I would tear it down and see how bad it is - then make that determination.
As far as parts availability, I have no idea.
I would need to know what specific parts it would need.
But as you noted, this set is bordering on old age.
RV sets are by no means inexpensive, but you have to weigh the possibility in that if you repair that problem now, something else may happen in a few months....
For the bearings, it could be either.
An NP66LP weighs about 215 pounds.