Standard install looks someting like this
Battery---Fuse--Switch (Master disconnect)---All the stuff that uses or charges the battery
NOTE: some stuff (like the explosive gas detector) may be hooked to the Battery side of the switch
Possible suspects:
Negative cable, not connected well to battery or chassis
Positive not connected well to battery, Fuse blown (or breaker triped) Switch off
Testing: using a TEST LIGHT (NOTE: there are 3 ways to test.. I usually use an automotive test light, get it at Wal*mart or any auto parts place.. But a 100 watt 12 volt "Trouble" light is an even better test.. I have one but avoid using it due to digging it out.. a Voltmeter is not a good test but can be used in a pinch)
Touch one clip to battery - (The actual battery) Tip to + Bright light = GOOD
Move clip to any unpainted chassis part, Still bright = Good
Follow the biggest cable from the Positive (if more than one) and touch first the cable lug at the battery, then the lug at the other end.. Bright-Bright = Good. whatever it's connected to touch the other big lug..
And continue out along that wire (not plugged in of course) till you get DARK.. problem exists between BRIGHT and DARK
But first;;;; Make sure the switch is not in STORE/DISCONNET/OFF (Aux battery may be the lable)
One final note: As you move out along the positive wire.. You will, from time to time, need to move the ground clip.. When you do this you should test with a known good 12 volt point to insure you have a good ground.. Another option is to "Bring the ground with you" (I have long coils of wire I can use for that)