Almost all my electrical quit working

Agreed, start at the battery and get your battery voltage with the multimeter. Then move to the fuse/breaker box and see if you are getting the same or close to the same as battery voltage. Find a good ground and start at the reverse polarity fuses and then move out to the other fuses. If you get no power at the breaker box then pull the front panel off and look for the wires coming in from the battery. They will most likely be near the fuses and thicker than the other wires near them. They will also most likely be white and either red or black. I'll bet that there is a marking on the board they are connected to that says either Batt + or POS or maybe just + where the red/black wire is, and a neg or - where the white wire is connected. Check these two wires with your multimeter. These two wires are the other end of the wires connected to the batteries.

You should have battery voltage at these two wires. If you do not then there is a break (fuse disconnect switch, or circuit breaker) between the batteries and fuse box.