My first choice would be a son or daughter's address to use. Remember, changing your permanent address effects more than just where your mail will be sent. It is what is listed on your driver's license, your registrations, etc. It could also effect your auto insurance policies and your health care insurance.
If a relative's address doesn't work out and you start looking at the mail service places, get firm estimates. Everyone talks about how great they are but they never brag about the cost.
We have used our son's address for over 2 years and planned on using it for another several years as full timers. Plans change and we are purchasing a winter home in SW Florida, but will still travel in our FW the other 6 months. Now we will be able to forward our mail back to his residence when we are on the road and he can get us our important stuff.