You can change them yourself but it requires special tools and a high pressure air compressor. You would probably spend more on the tools than paying the professional.
If you can jack the RV up, without it falling down, I would do it. I would get another person to help you. One of the things I wouldn't do is get under the RV for any reason, in case it might fall off the jack. If you don't feel comfortable, take it to a service station.