On long trips, it takes stamina and consistence to keep on top of FE.
It's easy to hypermile on the first part of the trip, and become tired -- then end up hitting the cruise, and forget about it.
So, the first thing I would recommend is to get prepared mentally. Do some facts and figures on 55 MPH vs. 65 up to 75. See what happens to time vs. fuel consumption and decide which is most important to you -- so you have the knowledge when the road starts hitting back.
I
do not recommend drafting semi-trucks for more than obvious reasons (several). They are good "Pace Vehicles" -- keep a safe distance, and make sure you can see the driver's mirrors.
I assume you're driving the van? If you have time, get some aero mods in there, change the oil, and get the vehicle ready. But at any rate, have a good trip and enjoy the drive
RH77