The HWH solenoids don't have a replaceable coil.
You can send your solenoid to HWH for rebuilding, or order a new one from them. At last count, it was 2-3 months for new, as HWH is only building to order to control inventory, but 2 weeks turn-around for rebuilding; rebuild was about $50 cheaper. I had this problem, and had some plugs made to seal the valve hole so I could use the coach for the 2 weeks. If you need one (or two?) you can PM me.
Baja-tested '08 2-slide 36'
Alpine: The Ultimate DIY'er Project