Curlysue,
Summary: There are lots of coaches available used. If you choice comes to low miles or more miles but used recently, pick the used recently.
Example: If you had two more or less equal coaches, one has 20K on it, but has been sitting for five years, the other has 40K on it but was driven 2~4K last year, get the drive last year.
No Matter How Old it is, if it shows signs of water leakage and it is a wood structure coach (most smaller are), do not even consider it. If it is given to you, it will probably cost more than it is worth.
I have a strange perspective that you won't hear elsewhere. I and many of my friends own coaches that are between 34 and 40 years old with proven mileages between 45k and 275k. Yes, they do wear out, but the big killer is sitting in the rain. The next big killer is sitting in the dark. In RV terms, driveline and chassis repairs are relatively in expensive. Repairing the coach is a different story.
No matter what it is, tires are good for 5 to 7 years. Belts and hoses are good for about 10 to 12 years and get the brake system flushed if it is over 10 years old no matter what.
Matt