Depends on your brand. Most either are Atwood or Suburbans. Most use a fixed thermostat system, usually in pairs where one is the operating temp and one is the overheat cutoff. In some models you can replace the existing thermostat with a lower temp one, but you have to check out your specific brand/model to see what options exist. Popular lore is that the RV water heaters run hot so that you mix with cold and thereby stretch out your hot water supply as most heaters are either 6 or 10 gallon.
Ed & Joie
2004 Vectra 40KD
Freightliner Chassis and Cummins ISC