I clean mine out the next day or so and then put it in the trash can. I would be surprised if it could do anything to a rash truck given the volume of liquids in the truck.
I dump mine in a small galvanized bucket until its full after many cooks then dump it in a compose pile.
Ash from lump is good for gardens, but I've heard that ash from briquettes isn'tI'd never even thought about doing anything with it besides putting it in the garbage can. Are ashes really good for a garden? I don't use much charcoal, but if it will help thing grow I'll be sure not to waste it.
I like to dump mine in a coffee can, label it "Grandma", and place it next to the garbage cans on trash day.