What Brand/Model of water heater....Suburban/Atwood/6 gallon/10 gallon/dual heat---propane only?
How long does it take to heat water back up....recovery time? 20-30 minutes OR is it less time?
How many valves do you have on backside (inside rig) of water heater?
1, 2 or 3?
Cold inlet (bottom valve) needs to be OPEN
Bypass (line that connects cold/hot lines) needs to be CLOSED
Hot outlet (top valve if you have 3 valves) needs to be OPEN
Also check outside shower....cold/hot knobs need to be closed and not just turned off at handle.
Answer the questions and better solutions to 'cold water' issue can be posted.....otherwise it's just a guessing game