I took a look at the owner's manual for the Empire Faucets RV Shower Valve w/ Vacuum Breaker # EM62FR and you are correct, the small plastic cap that you turned clockwise to increase the temperature is the temperature adjustment mechanism.
Since you've already adjusted the temperature on the faucet itself, then I recommend next checking your water heater.
A common component for a water heater to insufficiently heat the water is a faulty thermostat. The thermostat controls the water heaters... view full answer...
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