I think for any of the V8's your looking at 16 to 18 tops on the highway. Funny how fuel injection and more modern engine management seems to have advanced mpg ratings on everything but full size vans. Every older carburated van I had (and I had many, many) did about exactly the same as the new ones without the check engine light nightmares.
While I did have older V6 and Straight 6 Chev vans (I hated), I also did have a V6 powered Chev Express cargo (by accident) that would nudge just over 19 if you were really gentle with it, but good lord living with the lack of power in comparison to any of the V8's was just brutal. It was marginally better than the older versions, but yuck... I would never, ever deal with a V6 in a full size van again unless it was a twin turbo /supercharged something of at least 4 litres. Yet often, I come across V6 models for sale that some poor soul drove 200 to 300K.. I would have gone nutz. I do still look at them because they always sell for less, and I guess I am still young enough to consider an LT swap.
Somewhat weirdly, I do come across references that show the full size AWD Chevy's doing a little better on MPG's than the 2 wheel drives. I don't quite understand why, but I've personally experienced the same with AWD Astro's of which I've also had quite a few. LOVED the AWD Astro vans... always dreamed of finding an AWD full size.
Good Luck... I think you would be thrilled with any late model Chevy V8 Van. Don't put all your concerns on mpg's. While you have vans like the Promaster bragging about 20 mpg numbers, I have a '16 Town and country I occasionally pull a 2990 lb max cargo trailer with (same 3.6) and there is simply no magic there even though they claim it can tow 3600lbs. Sure it can I guess.. with the engine screeeeeaming up there around the 4000rpm mark and 13-14 mpg's. I'd get better with a full size and a V8.