electrical gremlins are back

Alright you need to start over and be incredibly specific

VOLTMETER.....What is this and how do you know it's accurate? Is this a dash mount gauge? What I'm wanting you to do is take some measurements with a DECENT accurate meter

The meter is hooked to the VR? You mean the VR IGN terminal?
You need to CONFIRM THAT by hooking a digi meter from BATTERY NEG to the VR IGN, does it actually show 10V?

NOW.........

"When I open the car door, the voltmeter reads 10v and drops down to nothing once I close it. Inside the cab there is nothing. No lights, radio, heater blower or anything at all."

You need to take some measurements WHEN THIS HAPPENS to find out where you are losing the voltage. Turn on the headlight switch, Maybe the key to "accessory" and the radio

Now start "wiggle testing" connectors, and start with the bulkhead connector. If that does not turn up anything, trace your (now modified) path from the battery through the bulkhead........Probe the two bulkhead terminals now feeding main power into the interior. You have voltage there at either/ both of those two?

Now go inside the car and probe the same two terminals, you have voltage THERE?

"All this time" you are taking measurements be aware if anything comes to life, the headlights, radio, etc.

I hope this gives you some hope: This / these problems CAN NOT be very complicated. There simply is not that much "stuff" between the battery and what isn't working

Also.............."what did you do" with the bulkhead connector terminals, to clean them replace them, etc?

YOUR TEST MULTIMETER is your friend. If you don't have a decent one, BUY one. ANY auto parts store, Home Depot, Lowe's Sears any hardware store sells decent meters. This is a CHANGE from when I "was a kid." In the 60's and 70's You could not just go anywhere and buy a decent multimeter.