So, the valve stem won't move at all? Or, did the valve stem move, but the valve disc didn't move with it? Those would be two different problems.
Not sure how it is with the 70's-vintage trailers, but from the 90's on the valve discs were made of Teflon. Standard maintenance is to lube the valve discs with silicone spray. If you're experiencing scenario #1, where even the valve stem won't move, try opening the drain cap and spraying silicone spray up into the opening and onto the valve disc (using those little extension tubes that come with spray cans is helpful). Also try spraying silicone where the valve stem enters into the valve body in the hope that some will make its way into where the stem travels, and subsequently loosen things up a bit.
If you're experiencing scenario #2, where the handle moves and the valve itself doesn't open, I'd say you'll need a new valve in its entirety.
Someone who knows more than me about this particular issue should be along soon to offer further advice.
Thank you for your service to our country, RetiredNavy!
Matthew and Lisa
TT: '02 T-2363;
TV: 2012 Ford F-150, 4x4/Off-road, 5.0L V8, "Big Gray"
Reese Straitline w/ dual cam