N7460N, OPERATING AS MID-AMERICA FLT 401, CRASHED WHILE ON AN IFR FLT PLAN ENROUTE TO CORPUS CHRISTI, TX. AFTER HAVING BEEN VECTORED AROUND A HEAVY THUNDERSTORM & CLEARED DIRECT TO WACO, TX. THE PLT CONTACTED FORT WORTH CENTER & STATED THAT HE HAD 'LOST MY ARTIFICIAL HORIZON & MY DIRCTIONAL GYRO.' AFTER ASKING THE PLT IF THE ACFT HAD A COMPASS & RECEIVING AN AFFIRMATIVE REPLY THE ACFT WAS INSTRUCTED TO TURN TO A 010 HEADING. SHORTLY AFTER, THE RADAR TARGET WAS LOST. INVESTIGATION REVEALED THAT THE PLT HAD BEEN EMPLOYED BY MID-AMERICA FOR APROX 3 WKS AS A NIGHT REFUELER. ON THE DATE OF THE ACC THE PLT WAS REPORTEDLY GIVEN A 1.2 HR PART 135 CHECK RIDE BY THE COMPANY ASST CHIEF PLT. THE VACUUM PUMP DRIVE WAS FOUND SHEARED WITH THE SHEARED ENDS POLISHED, INDICATING THAT THE DRAYING END WAS STILL ROTATING AFTER SHAFT SEPARATION. ONE VACUUM DRIVEN GYRO WAS FOUND SHOWING NO EVIDENCE OF ROTATION AT IMPACT. THE GYRO FROM THE ELECTRICALLY POWERED TURN & BANK SHOWED NO SIGNS OF ROTATION AT IMPACT.