I need to solder a new RV and some tech told me there was some type of coating that you could spray on the reversing valve that will keep it from over heating and not destroying the valve during the heat process.

Does anybody know what is used for that type of application?

Just use a heat absorbing paste or keep a few rags close in a bucket of water ring out the rags before you wrap them around the valve. Been doing this a while and always dread replacing reversing valves. Good luck!

You do not use solder. You use brazing rods usually 15% silver.

It is called a heat sink and there are several different brands. You still want to use just as little heat as you can to do the job.