Charging problem

I have a 1987 class A motorhome on a chevy p30 chassis. It has 103,000 miles with the 454 ci engine.

I have just replaced the starter, the alternator and the battery and the last time I drove it, I lost all 12 volt power and voltage gauge dropped all the way down to nothing. It seems as though the system is not charging. I charged the battery when I got home and it is reading 13.35 volts two days later while not running the engine. When I start the engine the voltage reading is 12.75 volts.

Any ideas?