Hi, I removed the gas tank on my k100. Drained the water, put in new gas. Fired right up.
Went to start it the next day and nothing, Power only to tail light, nothing else. At this point I have done the roll backward starter fix, Sprayed contact cleaner  and De Ox on Kill switch, Ground on Transmission, 4 wire connector under the tank and the terminals on the computer under the seat. Why would I have power only to tail light? Thanks in advance, Jerry

Most probably your ignition switch need to be dismantled and cleaned.
You find how to's if you search the forum.

Hi, took the switch apart, cleaned it. Found frame ground under tank. Cleaned them. Still nothing, Tail light and Radio work. nothing else. Any more ideas?

This is still pointing to the ignition switch, or you have more than one problem at the same time.
At the right side under the tank you find the connector for the RHS handlebar switch assy.
Measure the voltage at the contacts for the green wire when the ignition is switched on.