I bring the grate indoors, dump some soapy dishwater on it and let it sit 10+ minutes. Next a wire brush, then a steel wool pad with straight detergent on it.
There are never any wire brush or wool pad fibers left behind, all rinse away.
One or more times a season (as needed) I do the bottom/interior of the grill. I put degreaser on, let that soak a few minutes, and use a putty knife to scrape before rinsing that down the drain hole into an empty container (milk or juice jug, etc with a funnel in it). I dump the catch tray into the jug and let some of the degreaser run down into it to sit and soak before dumping that in the jug too. I use water conservatively so I need only a gallon or so jug capacity before the runoff is clean enough that I can let it go into the yard without killing the grass.
The heat spreader plates above the burners are just covered in ash and are cleaned dry with the putty knife, mostly to reduce flare-ups.
So far, so good. I've gotten over 20 years out of my current propane grill, mostly thanks to a cover that keeps rain and snow off so it hasn't rusted away. I've had to replace the cover at least twice.
You guys are grossing me out talking about heat as a cleansing agent. My grate is clean enough to eat off of before I fire the grill up.