This circuit is simple. The only thing between battery power and the lights/gauges is the main fuse, the main switch, and the individual circuit fuses. Not likely all fuses are blown for every circuit and you said the main fuse and main switch are good. Power and ground wires from the battery are the only thing left. How do you know the cables are good? Just because they are tight does not mean they do not have corrosion between the terminal and the battery post, causing resistance. The ground could also have resistance where it connects to the frame. I have seen some crazy things in electrical circuits over the years. I saw a landing light on an aircraft creating a problem in the glide slope years ago. Totally two different systems, not even related. Electrical flux from the nose gear landing light was wreaking havoc on the glide slope function of the nav avionics in the nose of the aircraft. Crazy, never happens, but it did. Take nothing for granted. Check power coming out of your main (ignition) switch. Should have 3 circuits on the outlet side of the switch. An indicator of the integrity of your battery cable connections would be to check for power on the red/green wire going to the meter. This wire feeds the clock and odometer for memory. It comes directly from the battery, only passing through the main and backup fuses on the way. The symptoms you are describing are classic ground fault symptoms. You need to check both battery cables thoroughly from terminal to terminal. Don't just check to make sure they are tight. Take them off and clean them. I don't see anything else it could be other than the main fuse or main switch, both of which you say are good.