I hope we're talking about the same part, the splitter, pressure differential switch below the master that sends fluid from the master to both front wheels and the rear diff, if so. At the shop we used to cut off the line and use a 6 point socket on the nut, but if you've been in there with vice grips you probably beyond that point. Do not heat the valve, if memory serves me correctly the body is brass, and will transfere heat and burn out the switch seals. Can you remove the valve from the coach and work on it in a vice ?? If you can get it to a vice and blow all the fluid out of the valve , you could try heating the nut with a soldering iron, that's about all the temp the valve will tolerate. Or look closly at the valve and see if the switch is removable, to prevent damage.