OsiriX was also one that I usually suggest first. I looked at their web site, but other than the fact that OsiriX specializes in viewing medical images, I don't see ANY mention of dental images, in particular, no mention of a file type like .rvg.
You could email their support folks to see if the free version of OsiriX will do that.
That file format (.rvg), amazingly enough, is a limited-use format used for dental X-ray image files, using Kodak software, and I could find very little about those files.
One suggestion would be to get your dental office to convert them, as the imaging software probably will output the images in other formats.
I did only a quick search through the usual suspects for file viewers, but the only mention that I could find about .rvg dental-image files was Kodak itself.
I DID find a couple of questions similar to yours, looking for file conversion, but nothing more than (unhelpful) generic answers to look for file conversion software.