The back of the counter is held up by a piece of angle aluminum which is riveted to the wall with rivets (extra large heads on these rivets).
It's likely that one or more of the rivets has worked loose and the angle aluminum is of the wall allowing the counter to sag.
To repair, remove the stove and the countertop. You can then level the angle aluminum, drill new holes and replace the rivets. You can get these rivets ($30 for a box of 100) at Grainger or other industrial supply house. They are pop rivets so you'll need a pop rivet gun ($20 - $50).
1986 Airstream 345 Classic Motorhome