Something is shorted in the circuit/s. Something, maybe wired backwards?
Disconnect shore power and start a process of elimination.
Look for loose wires and evidence of sparking, in or around the fuse panel.
Any screws or nails installed where they may have shorted wires?
Any places where ice or water may be shorting the wires?
Anything plugged into a 12 volt outlet? A 12 volt hair dryer with an internal short, could cause fuse to blow in the panel.
Your post sounds like the problem is with multiple circuits. How many and which ones? Is it all of them?