Hi everyone, thanks for reading and offering guidance. 

I have a new-to-me 2016 Scamp 16 and I'm trying to figure out the systems. 

When I am connected to shore power the lights, fan, pumps all appear to function as intended. I am aware that when disconnected from shore power, the onboard battery can only run the 12V components (ie, lights). 

When I disconnect from shore power, the 12V components no longer function. Voltage at the battery terminals reads 12.4V. I've replaced the 20A fuse on the positive. Voltage at the fusebox reads 12.3V at each of the fuses. I've tested all the fuses and they're not blown. 

Any thoughts on what next steps should be?

Thanks again.