99 astro and Trailmanors

Uh.., just checking JP. Did you read the owner's manual for your Astro Van?
I have a '96 Astro and the manual specifically states that one should never tow with the overdrive activated (D range on the gear selector). It says that doing so will overheat the tranny.
I tow our 3124 in 3rd, as recommended, and get 15 -16 mpg. Sometimes I'll slip it into D on a totally flat stretch just to lower the rev's some but I notice the strain on the drivetrain when I have to push the accelerator to go uphill.
Towing in D causes the transmission to "hunt" between 3rd and 4th gear all the time adding stress to the engine and transmission.