It sounds like they'd have you towing at about 50-55mph max.
Which truthfully isn't a bad idea to be keeping your speed low while towing.
You probably would find the best efficiency at about 50-55mph, but do you really want to be going that slow.
This is a very technical question that I'm sure others on here could answer for you, but I can't help but wonder if going 10mpg slower while towing with a diesel is going to make much of a difference. At least a difference that justifies dropping your speed like that.