The shut off valve on my permanent propane tank leaks a little. Is there a reason that I should not use a valve off of a portable 20# tank? If so please explain because I can't think of any reason. The RV is a 25 year old class C.
Is this a horizontal tank? If so, it is a different valve than on a 20# vertical. All the reason you need. You probably don't want to hear it, but if the tank is as old as the RV, it is probably time to replace it. Propane is nothing to mess with. Get the right valve or a new tank.
I second Bill's suggestion. Don't mess with an old tank. Also, the valve on a portable 20# tank may not work on your tank. IAC, you need to have an OPD valve (Overfill Prevention Device) that shuts off the incoming propane when the tank is 80% filled.
The valve from a vertical [DOT] 20# tank will not function correctly in your motorhome tank because the OPD mechanisn is oriented the wrong way and calbrated for a different size tank as well. The OPD, which is basically a float valve, has to be designed and calibrated for a specific tank configuration.
There may be other differences as well, but these alone are suffcient to disquality the DOT tank valve.
Thanks for your replies! I did a search online and found the proper valve for my tank, $60 total. You guys are right I'll sleep better knowing it has the proper valve. I didn't replace the tank because when I took it out and repainted it about 10 years ago it only had surface rust on it. This time it was the same way.