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.

Start at the battery. If you have a good connection at the terminal you disconnected, originally, all your lights should be on and bright. 12v is necessary at the furnace control board and thermostat to make the heater function.

I have a multimeter. I'll check the cables when I go to the farm Friday. sure want to get that heater going.

I would give a double check that the battery cables are correctly hooked up. Usually it's black or red to positive and white to negative. But don't just take my work for it. Go grab each wire one by one and hand trace it back from the battery. The negative wires should all go to the frame. On the positive wires check any fuses or circuit breakers on the wire are good.

Check at your power panel and verify that the rev polarity fuses (probably 40amp) are good.

Do you have a multimeter or 12v test light?

I checked all fuses and they were fine. I wondered if I did that, but the water pump also worked so I must have gotten that right.

All items listed are 12 volt or 12 volt controlled.
I would start by plugging in. Check for power, 120 volts AC to rig and then to the converter input. No 120 volts, reset 120 volt circuit breakers in the rig. Turn all breakers off applying light pressure toward off. Then turn main breaker on followed by each branch breaker. Verify that the GFCI receptacles are working. Push test button and then the reset button.
Once you know you have power there Check for 12 volts DC in the DC distribtion panel, normally located adjacent to the 120 volt citcuit breakers.
Part of lights working, could indicate blow fuses in DC distribution panel.
Look in DC panel for two main fuses for panel.

Any chance you hooked the battery up backwards and blew out some fuses?