When I turn the key switch to ignition, engine switch to Run, and Start ... everything goes dark.
Trickle charger shows battery fully charged and it's not too old. Switch everything off and back on and, nothing. Totally dark.
First off, take the battery out , load test it, or take it to a shop/store that can load test it. If after testing it shows good, then reinstall battery checking and cleaning all battery connections (both ends of cables). If it test's bad, then it's self explanatory.