I know you said you are buying a new battery but wanted to add my 2 cents. I once had a battery that would pass the tests and in the morning was dead. I asked the parts guy to charge it up and test it and then let it sit all night and test in the morning. It passed the test that night and failed the test in the morning. This sounds exactly like what you are reporting on your battery.
If you are mostly or always plugged in when camping then get a cheap 12 volt lead acid battery and go on with life.
If you dry camp often and have the room then put in two 6 volt batteries from Costco or Sams Club.