Good Sam RV Club Member
If it's the thermostatic mixing valve made by Honeywell, they can be disassembled, cleaned with CLR and re-installed. I've done this twice with mine but I carry a new one just in case. The one normally used is a Honeywell AM100-1 and can be purchased online for about 90 bucks. Aqua-Hot wanted three times that so I decided to fix mine or find a different source.
2008 Monaco Camelot 40PDQ, 4 slides, 2012 Dodge Ram 1500 Hemi Quadcab 4x4. Blue Ox, Garmin 7200 GPS, Doran Pressure pro on all 10, M&G Brake, 50 amp SurgeGuard, FMCA, Coachnet.