Stuck calipers are very common on class c's, because water from the cabover drains onto the front brakes, and the lack of use makes the sliders seize.
That said... I don't think that's what's going on here based on your events.
I'd very strongly suspect the back brakes aren't doing their share of work. Your E350 has rear drums. The star wheels rust up and no longer self adjust. Also, many class C owners never brake hard enough going backwards to even turn the adjuster wheel in the first place.
If it's not super rusty, you might be able to turn it by hand. But at the age you your coach, you probably need new adjusters, and possibly other drum brake hardware regardless of shoe life.
If the shop who did the front brakes was asked to do a full brake inspection, and they didnt' acutally check the rear brake hardware, IMO they should give you a break on the front brake costs.