A quality job or sealing with EPDM caulk or Eternabond tape is not a concern. Rv roofs (rubber or not) require regular attention to seams and openings, so you have to expect to find caulk and/or tape on those joints.
An EPDM rubber roof is vulnerable to tears but those are easily fixed with caulk or tape. The main concern on the rubber itself is plain old wear. The rubber oxidizes and wears away, leaving just the backing material, which is sort of a canvas. If you can see the weave of the material showing through, the roof material is shot.
Another poor sign is loose rubber sheeting, aka "bubbles", where it has come loose from the luan substrate. It's easy enough to glue it down again, but bubbles suggest that there may be a problem underneath that is loosening it, e.g. water intrusion.
The EPDM rubber material itself is normally warranteed for 10 years and the typical life span is probably 12-20 years, depending on a variety of factors. Acid rain, excessive scrubbing, or use of petroleum-based chemicals will shorten the life.
A worn rubber roof can be replaced (expensive), or re-coated with liquid EPDM. A re-coat is under$200 if you do it yourself, but can be pricey to have an RV shop do it because it is a day or so of labor.
On any RV roof, even a fiberglass one, the thing to look for is signs of water intrusion. This involves inside as well as outside inspection. This is the main thing you are looking for on any RV.