1. Need year/brand model RV. Need Brand and model of water heater.
2. The "lighted" switch does NOT indicate the Water heater is running on 120. ALL it does is indicate that 120 current is passing thru the wall switch to the Water heater 120 connections. You still have a problem with the 120 side of the Water Heater. Need to know Brand and Model of Water Heater as the only 2 brands function differently on their operation on 120.
3. Since you use the 120 exclusively, the odds are the 120 element is indeed burned out, but there are a few checks you need to make before you determine that. Doug