I'm not rationalizing my travel speed. In my OP I said I have a schedule to meet. So I meet that schedule, which requires me to drive pretty much at or above the posted 70mph speed limit. I'm just trying to get better than the 12mpg that I started with. I have the time and ability to mess around with aerodynamics on this $500 van, but not the $30k to buy a Mercedes Sprinter diesel.
In my experiments I found that (at the time with the mods at the time) I got about 15.4mpg at 60mph, and 14.7mpg at 70-80mph. I'm fairly certain (without hard data) that the relatively low delta in FE of 5% has a lot to do with being "in the pack" at 75mph. FWIW I give 2-3x the draft clearance of most drivers here, enough so that there are always people changing lanes into my clearance gap. Even with the suspension and tire upgrades I've done, I know that van is heavy and not Miata-agile.
Regarding the fans, even with the mechanical fan and no shroud the engine coolant never goes above 200F. The radiator inlet temp is about 185F in worst case, while sitting in traffic with the A/C on full blast. Most of the time on the highway the inlet temp is around 150F with engine temp around 180-190F. I'm saying this based on best estimates of engine heater core temps checked on various points of what each letter in "NORMAL" means. Before I try anything else I'll install a real temperature gauge for the engine temp, radiator temp. Thanks for the suggestion on the oil temp, I hadn't considered that before.
For towing I should probably put a temp gauge on the A/T fluid too, though with a decent cooler it's unlikely to go too high. The Ford racer guys say that the B&M coolers are made by Hayden, but much more expensive. I'm not sure if that's actually true, but the Hayden 676 ($36 from Amazon) does look pretty close to the B&M 70255 ($75 from Jegs). I'll have to look into that. The stock setup goes into the bottom of the radiator, which makes it more of an A/T fluid heater than anything. A C6 tranny should be ~150F, not 200F...