Your WH probably has a check valve on the outlet. If that valve sticks, it will prevent/slow the flow of hot water. That maybe your issue. After sitting all night, the valve sticks. Then, after hours of some usage, it eventually releases completely and a normal flow happens. 45 psi may also be a bit low which would make freeing the stuck valve more difficult. Just a guess.
Also, make sure your cold inlet by-pass valve is full open so full pressure is entering the tank.
Larry, Eileen, and Finley
2004 Alpine 36FDDS
Third motor home, first Alpine, no need for another.