Just throwing an additional line of thought into this discussion (before it even gets going, ha....!)
Regardless of the measurable additional "work" that an engine has to do in order to get a larger/heavier set of tires rolling / maintain their speed (and overcome the additional rolling resistance.....)
....If you've geared the final drive ratio properly, then the additional work the engine has to do shouldn't add up to THAT much additional strain on the engine/transmission. (To some other parts of the van, the story is different.....)
Sure, the van might not ACCELERATE quite as quickly.
(Since it has to "spin up" and accelerate the additional rotational mass of those big tires.) But once the van is "up to speed," (again if you've geared the van properly to compensate for the taller tires), then the engine should be able to maintain speed (and tow) pretty much the same as it would with the smaller tires.
Again, the biggest noticeable difference will be in how fast the van can accelerate itself. Maintaining similar amounts of cruising speed shouldn't be a huge difference however.
Two areas that should DEFINITELY
come under scrutiny as you continue to "size up" and head towards taller tires:
2) Strength of rear axles / axle shafts.
Even with a re-gearing (actually, ESPECIALLY with a re-gearing....), the axles will be under increasing amounts of acceleration-driven torsional (twisting) loads
with the taller tires. And brakes will lose some of their effectiveness/stopping power
(as the taller tire is imparting even more leveraged force to the brake rotor as you attempt to slow the van down.)