You'll find a lot of information on-line. I looked for certified vehicle appraisers in my area. There were a number locally who advertised doing diminished value appraisals and a few of those who do RVs. Then I did some research on customer satisfaction of prior clients.
The company I hired did the inspection, wrote a detailed report providing analysis of fair market value of motor home prior to, and after, the collision, and provided a sample demand letter to send the insurance company. They also offered advice regarding how to proceed and what to expect. I believe I paid under $800 and it was some of the best money I've ever spent.
If there's anything I can do to help you feel free to pm me. Good luck!