Air bags

Look them over for cracks and wear marks. If you see rub marks, find what is rubbing, and fix that. The bags are pretty thick.
When I first got my last Pete, the bags had some cracks. I bought replacement, but did not have time to change. Put 1 in box to take with, ran over 10000 miles before I got them all changed.

Last year we had a rear air bag with a slow leak in it. I took it to a Cummins repair facility and they replaced the leaking bag. I asked about replacing the other rear bag since they were both the original. They stated that they usually don't replace them until they start to leak. I though that was odd, but it's been over a year now and the older bag still has no leaks.

I don't think newer bags will give you an improved ride. I have no direct experience so maybe I'm wrong but am just thinking of the physics of it. Air is air and if the bag is holding, what else in there?

As for running with old bags I've heard of a some folks traveling with a spare bag or two in case of one breaking while on the road. I think it's a lot easier to find someone to replace a bag than it is to get the bag to your location and have it installed.

At or close to their life. As Wolfe stated, Check them. depend on your condition.

When you can see the cords showing through or chunks of the outer layer flaking off.

Yes, if you plan on keeping it for a long time and prefer to do maintenance at your convenience instead of on the road, do it when it suits you.

But, I know of a number of 20 year old coaches with original airbags.