my 2002 vw rialta when key urrned on to start engine it wont engage the starter

Need more information. Do the dash lights come on? Does the starter make any noise? Does the engine make any noise? Possible culprits include a bad starter motor, a bad starter solenoid, a bad safety relay (which is the switch that makes sure the vehicle is in park or the clutch is depressed before starting), bad ignition switch, and loose wiring. If it has been making a metal on metal noise prior to turning over, you probably wore down your gears.

I had a very long-term intermittent problem with a Eurovan Camper (also Winnebago) where the starter did not work when warm if the battery voltage was at all low. It would start fine after waiting about 45 minutes. Over the years, the problem got worse so that the waiting time was MUCH longer. After MANY trips to the mechanic, they finally found the problem: one of the interlock switches for the front seats (so the car wouldn't start if they seat wasn't in the forward position) was heat-sensitive and its resistance increased as the temperature increased. Replaced this, then no problem.