I would guess the fix is pertinent to why the tire is flat. If it's a nail hole or something similar, then someone can come fix the tire. But if the tire has blown, thrown a skin, blown out a sidewall and/or is otherwise not able to be fixed, then it becomes a more complex issue. A new or similar sized replacement tire would need to be located and paid for, and probably shipped to the closest tire dealer of your choice or your roadside assistance firm's choice who could come put it on the wheel. Hopefully the tire dealer choosen would have a mobile repair vehicle including the expertise and heavy duty jack to lift a heavy coach, and have pneumatic equipment to remove and retighten the lug nuts.
Another piece of this "fix" could be repairs are needed to the undercarriage and/or siding of the coach due to damage from the blown tire, or from running off the road into a bar pit, guard rail, trees or another vehicle. In this case, your roadside assistance firm necessarily becomes your guide.