Trace the feed wire from taillight back to it's origination, testing for 12V at every connetion or splice until you find power.
Have you tested all the light bulbs for continuity? Although improbable, they can all be blown for some inexplicable reason.
2000 Winnebago Ultimate Freedom USQ40JD
, ISC 8.3 Cummins 350, Spartan MM Chassis. USA 1SG, retired;PPA,Good Sam Life member."We the people are the rightful masters of both the Congress and the Courts - not to overthrow the Constitution but to overthrow men who pervert theConstitution. "Abraham Lincoln"