Hello Brian,
As the cold weather approaches you must ensure your battery remains fully charged at all times; otherwise electrical gremlins will start to appear.
If you don't use your van every day, set up a trickle charger to keep it topped up. I use a CTEK.
( NB. The CTEK will perform an alternator test, although I don't know how 'good' that test is).
Batteries need to be tested under heavy load, otherwise the output voltage will be much higher and misleading.
Halfords or Kwik Fit can perform the test. Probably at no charge.
With a new battery and a new alternator, it seems safe to assume the 'problem' lies elsewhere.
Ensure all your battery connections are tight; no movement at the terminals between leads and terminal.
Check for any loose earth leads.
Ensure your room/mood lights are off.
Check all your lights, in the dark, and ensure they all all fully off.
Does the clock need resetting, or the windows after these issues ?
Good luck.
If you do have a camper, get the leisure battery tested (or disconnected).