I can't recommend anyone to do the job but, it's simple. I suspect that for your rig even a place like Discount Tire could do it. It's just a matter of jacking the wheels off the ground and pulling that outer wheel off. I did my own a couple of years ago and it didn't take very long. I jacked the axle one side at a time, pulled the chrome covers, pulled the outer wheel, screwed the extensions on and put it all back together. I did have to drill the center cover and rivet on two brackets for each side. I used the steel braid extensions. Make sure that the installer confirms that the valves are snug. I had one loose and, but for the steel cap it would have been a real problem. I just snugged it down and everything has been fine since. The hardest part is the jacking the motorhome. I always hate jacking up that much weight. I have a air controlled jack so I can slide it under the motorhome and jack it up without being under it in any way. Once it's up I slide a heavy jack stand under there.
Anyway, if you take it to a tire store that normally does cars and light trucks be darn sure they don't cross thread your lug nuts. They did that to me on one wheel and it became a problem. When I need repairs I try to find a heavy truck repair place. They always do the job right.