Battery drain after starter motor change.

Hi all.

Racking brains here as to what could be wrong.

Changed starter motor last week (what a pig of a job).
Van was running great then today, went to start and battery was flat.(new battery).
Stuck another battery on and it started drove a couple hours then when went to start again that battery was flat.
It's like it not charging, so it's pointing to alternator, but no lights on dash.
Just wondering if it could be an earthing issue.

Many thanks.
Stu.