Gauges went crazy after dead battery - HELP!

Sounds like an electrical problem that involves one of the computers.

Did you check for any OBDII codes? I'd guess you have some code that needs clearing.

BTW, in the photo, the tach isn't working. Is the motor running in that photo?

Go through all your fuses. Actually measure them with a volt meter to verify they have voltage going though them on the little tangs on top of each fuse. I had a similar issue and it turned out to be a relay that hung up due to too low of voltage that affected my anti-lock brake system. Once I reseated the relay and voltage was restored, the lights went out, the codes stayed cleared and the tach started working.

1. check for any damage from rodents
2. charge the battery to full
3. start the engine and press the gas pedal just a bit this will engage the voltage regulator to start working
4. drive the RV at least a half an hour, this will give the computer enough time to regain all of the parameters that it needs to run the engine transmission and the dash gauges.

What happened is that the computer memory used the available voltage to maintain the memory, at a given low voltage the memory goes blank, and you basically reprogram it by driving, normal with the new computer controlled vehicles.

navegator

you computer is in need of a reset. you may have to take it to ford or chev to get reset.

No rodents.

unplugged ScanGaugeII and all is well. Plug ScanGaugeII back in, and problem persists

Are rodents eating the wires? I've had trouble with that in both car and RV. Make sure everything works before you drive it, just in case something ate non-visible wires or whatever.

It may be complaining that there was very low voltage. I would drive it a bit and see if everything goes away.
I'd also charge the battery a lot more.