Generally you need to name any regular drivers who aren't your spouse or child. "Regular" is subject to a lot of interpretation, but [for example] you don't need to identify someone who helps you out by driving it a home when you cannot. Letting someone borrow it for a few days or weeks probably puts him in the regaular driver category, but only your insurer can answer that.
I don't think they can deny you the listing of another driver who is not a family member, but if you say the word "rent" or even "borrow" without you present in the coach, they probably have the right to decline coverage and maybe even cancel. Unfortunatey, the only way to find out is to ask.
I would approach it by stating that my good friend drives the coach at times and ask that he be listed as a regular driver. That gets him on the policy. I leave it to you whether to advise them if he will be the sole driver for a few days or weeks.
Former owner of 2004 American Tradition and several other RVs
Home is West Palm Beach, FL