Welcome to the Forum, congratulations on your acquisition.
I had a similar problem on my '77 Safari.
You can get a valve that attached on to the sewer pipe connection, I used one as a temporary measure until I could replace all the valves.
This is the one I used, you'll need a Valterra to Thetford Adaptor like this one
when my drain valve didn't open, the handle broke off, if the valve is opening but not draining any faster than with it closed you likely have a clog and the valve isn't closing fully. Or it may be that you just don't have enough water in the tank to make it look like it's fully draining.
Here's a link to my Bathroom and Plumbing repairs/renovation thread, it may help when you start taking things apart.