If measuring voltage on the same 15amp circuit that the 12.5amp elect heater is plugged into, there will definitely be voltage dropped on that circuit. A 15amp circuit that uses #14 wire and has several receptacles between the heater and the circuit breaker is a hazard unless all the receptacles are wired pigtailed. The common method of wiring receptacles in RVs as Myredracer shows uses a 2 piece compression receptacle that has very little contact surface with the wire. Under high loads(heaters) the small contact area gets hot causing metal expansion and loose connections which then causes even more heat. People, these outlets are JUNK! I've replaced mine with 20A with screw and pinch plates and wired them using pigtails.
