There are a few RV driving schools around. It all depends on if you're close to one or not.
RV Driver Training: It's Worth Going Back to School! | Fun Times Guide to RVing
Supposedly you can contact various truck driving schools and talk to them about it. You might be best off talking to one about their instructors and if a specific instructor would be willing to do something one on one.
I know your pain! I went from a couple of summers of "towing" a kayak trailer from a parking lot down a walking path to lake side and back. Then I got a pop-up camper with a Durango. And then we went and got stupid buying a 42' fifth wheel and crew cab/long bed truck. I've caught a couple of curbs (ironically, 2 at slow speeds in the last month
The best thing (and only good thing) that my dealer did for me was sent one of their techs on a test drive with me. He sat shotgun while I drove for 30-45 minutes. He gave me tips and things to watch out for. It gave me a ton of comfort.
I sucked at backing up for a long time. Still do at times.