If connected to shore power, check the 120 volt input to the converter. Check converter output for 12+ volts DC. Check the 12 volt DC distribution panel for 12 volts DC. Look for a BAT fuse in 12 volt DC distribution panel and test it.
What is the DC negative buss bar made of? If steel, you need to disconnect all the wires, clean with strong steel brush, coat the connections with a good quality electrical de-oxidation compound and reconnect.
Test the battery connections. Put an Ohm meter lead on the post of the battery and the other lead to battery cable. Using lowest scale on meter, it should have zero ohms of resistance. Do this for each of the negative cables.
Switch meter to volts and test the battery positives in the same manner. Positive leads should have zero volts.