Hot water heater drain plug leaking

Pick up a 1/2" pipe nipple from your hardware store......
Screw it into the drain hole.....easy goes it---screw it in a little then back it out then in/back etc. (chasing the threads) tank is aluminum and the pipe nipple is steel so it will clean up the threads.

Then get a new 'nylon' plug.....they come two to a pack

Screw new plug in hand tight then snug up with a wrench.

Fill tank/check for leak/heat water /check for leak.

"plastic bolt". That means you have an Atwood water heater. Go to your local RV parts dealer such as Camping World and get a new plug. In fact, get a spare too. I'm guessing that a new plug will solve your leak issue.

Would caulking the end of the plastic bolt work? I just have no idea how to chase a thread

The previous owner may have buggered up the threads. Given your description I would try chasing the threads to put them back to normal so the plastic plug seats as designed