It all comes down to the road surface. 25 wh/mi is an easy number to hit, even in the mountains, on paved roads. I'm not guessing, I live in the rocky mountains.
But if your tire is sunk into soft dirt, sand, mud, as much as 1-2 inches, then your wh/mi can double or even triple. 60wh/mi totally normal, ( with a 1000w bike traveling slow) with a fairly wide "normal tire" if the surface is soft sand. A pipeline road, to me, sounds like it could be all churned up if it's been dry a long time. Or, the opposite, have long stretches of mud in every low spot. The 250w limit will help your efficiency some, but if the going is tough, then you will be pulling the max watts and going 3 mph. This could call for a mid drive to improve efficiency, but still end up with a pretty high wh/km result.
If the road is packed dirt, then you could end up 20 wh/mi or less.
You'll need to scout that route, to have any idea what it will take to do the 180 k leg. In any case, 2000wh needs to be your minimum target battery size, unless your riders are pro, or extremely strong athletes. Ride some sections, get some real world wh/km numbers to work with.
If the riders are that strong, big deal, Sagan could ride the leg on 250wh. Who cares how far a pro athlete can pedal a too heavy bike?