I rent vehicles all the time -- a few times, large vehicles with a heavy payload...
I'm thinking that the truck isn't going to give you the 21 mpg predicted. That's really stretching it.
Second -- STAMINA.
This is a long trip with several temptations to say "to hell with it, let's just max cruise and get there". Been there. It's tough. You and the other drivers may just go the limit or governed limit after a while. On my 15-hour move a few months ago, this exact situation happened. I got weak and just set the "cruise" to full-throttle governor (75 MPH). The trip should have taken 11-hours, but traffic jams offered otherwise.
Sounds like you need a driver
When's the event?
The rental truck agencies are 18 y/o and older (Penske is my preference -- a bit more than U-haul, but worth the piece of mind). I've rented at least 3 times with moves with Pen. I had a Diesel Freightliner that averaged 18 hwy with a hydraulic lift instead of the ramp -- very convenient. No trailer, and enclosed box. See if you can insist on Diesel if you rent. The price of the fuel is outweighed by the FE.
I'm late to the party here, so dunno if this helps
RH77