Old-Biscuit wrote:
Open lower cabinet doors and blow warm air into cabinets
Don't forget to open the access to your water pump and the cabinet hiding the backside of your outdoor shower (if you have one).
I too, have had a frozen dump valve (more than once). What worked fastest for me, was disposable hand warmers (sometimes called "hot pockets"), available at many sporting goods stores, and probably Walmart. I wrapped them around the valve, then wrapped it with a towel to help contain the heat. The hot pockets also free you up to work on multiple areas at the same time.
If you can get at the check valve on your city water hook up, those pocket warmers may work on that too.