Check your 12 volt fuses. Sounds like a blown or dirty fuse on solenoid or relay power. Typically in the era of your coach these were inline fuses frequently in a 12ga line coming off of a + post on one of the chassis batteries. If they were in line to close to the top of the battery they would get slobbered up from battery out gassing and the connection would be so poor conductivity was lost. Seldom was the fuse blown. Anyhow this completely shut down the drivers ability to start the coach but it could be gotten around by a jumper just as you have described. Many coach builders will place a gang of wires on + and - terminals as a convenient place to pickup power. Check it out. Good Luck.
