I have stored my RVs in several different storage lots since 1978. We use our RV on average once a month for weekend trips. Once every couple of years a long trip of 3000 to 7000 miles. It's rare that we put more than 400 miles on the RV per trip. I am amazed at how many RVs in the storage lot get used once or twice per year.
That being said low mileage RVs are not unusual. Most folks tend to stay close to home due to other commitments.
The best way to judge the use of an RV is to look at the brake pedal. The rubber pad will begin to get thin at 60,000 miles & wear through at 80,000 miles. At 20,000 miles the rubber pad will look nearly new. Floor mat on the driver side (if equipped) will also indicate use. On all of my vehicles the floor mats had spots worn where the heel of my shoe rests.
On a Trailer I pay attention to the carpet/flooring and fabrics on the chairs & couch for signs of wear to indicate how much use.