4.6L Mercury Mountaineer w/ TM 3023 - Fuel Economy?

We towed our 2009 3023 with a 2002 3.8L V6 Windstar for one season, until it ceased communicating with the electronic brake controller. We just bought a 2007 Mercury Mountaineer with 4.6L V8 engine, 7000 lbs tow capacity. We just towed it for two hours to the TM dealer for some service here in Michigan, and were surprised that the fuel economy dropped from 19-20 with no TM, to about 12 with TM in tow. We did not notice this dramatic of a drop with the Windstar.

Can anyone else comment on their experience with a Mountaineer or Explorer?

Randy and Mary
2009 3023
Tow with 2007 V8 Mountaineer