Although most campgrounds and RV parks do have in their rules that maintenance and repairs are not to be done on the site, as has been said, most places are pretty flexible as what they don't want is the place to appear junky with vehicles up on blocks for days on end and that kind of thing. Normal maintenance and repairs that can be done in short amount of time are usually overlooked.
After all, sometimes it's necessary. If the coach has some type of malfunction so it can't or shouldn't be moved and it's something that you or a mobile service person can work on, just about any RV park will allow work to be done on site. So how can they determine what is actually necessary to be done on site and what is maintenance that could be done somewhere else unless they are out there observing all the activity that's going on?
For instance, we had a mobile RV repair service out to replace hydraulic hoses to the rear slides as he said he could do it in one day on site and would clean up any mess that would be made. As he was working on the hydraulics, DH got under the coach and changed oil so they were both under there doing stuff. Who knew exactly what who was doing under there? If somebody from the RV park had come over and asked what was going on, all DH would have to say is that the coach could not be moved and repairs would be needed before we could pull up camp and he was under the coach just observing what the mechanic was doing.
But I don't think the RV park would have cared either way.