Welcome to the forum. There are four types of vents on your roof. First is the refrigerator vent, it's about 9" wide by 24" long. Second is the "roof vents" which are 14" x 14" and may or may not have a fan and a cover over them. Third are the plumbing vent's which are about 4" in diameter and finally, on some RV's (on your Montana) are "attic vents". The vent in your picture is an "attic vent". The purpose is to ventilate condensation/moisture out of the "sealed attic space" during use. There should be one on each end of your RV roof, to set up a "flow of air" through the attic space, helping to dry the air in that "dead area". Your vent, in the picture you posted has the "cover" missing. It should look like the picture I'm posting in this response.
It seems that most plastic roof attachments are "designed to fail" with UV rays, hail and age, so they are intended to be "easy to replace". Just how easy depends on how you can access the roof, how simple the job of removing old sealant around your vent stack and how much care you use not to damage the TPO/EPDM roof membrane during the job.
The vent in your picture "should be available" at most RV parts stores, but it may not be "in stock". Since it's not used on all RV's, it may need to be special ordered for you.
Here is one source for the vent:
http://www.shop.classarvrepairs.com/...776-102776.htm
NOTE: The vents in your picture are, I believe, the old style. The new caps are "covered" and have a "grid work" instead of the holes. They less prone to leak during rain storms and prevent flying insects (wasps and dirt daubers) from entering.