Our 2002 E450 had stuck rear brake calipers before we bought it in 2008 with just over 30,000 miles. Not long after, say 35,000 miles a front caliper stuck and I replaced both. Past fall, at about 40,000 miles, a rear caliper stuck and I replaced both. Past spring, I changed out the front axle mostly to get newer bigger brakes, so I can't say how long the front calipers I'd replaced would have lasted.
I think the frequency that an RV is used has much more to do with things like brakes (and tires!) than age or mileage. We try to take ours on a local camping trip, around 100 miles round trip, monthly or so. Just running the engine a few minutes doesn't get it.