I had a similar problem,I though it was a drain, but it was a charging problem. it turned out to be a bad earth. I cleaned them all up and it's been fine ever since.
2002 td5 90.
I got between 29-32 mpg when I got it on standard boosts and 235/85 general tr's.
With a 2inch lift and 285/75 Cooper st maxx, I now get between 27-30.
I mainly do shortish journeys between 5-15 miles in length .