I have a 89 Chev motorhome with a 475 P-series transmission that will not shift into drive. The fuel level is correct with no discoloration or burned smell. It will shift from first to second at the correct speed, but never into drive at any speed. The linkage is tight and seems correct because I can manually move through the gears all the way to neutral, each at the correct spot, it's just like there is no Drive...... Any ideas? Thanks to everyone in advance....