Well... plenty of issues to be clarified.

- what trucks (brand, type, capacity etc...)?
- what application (loads, surfaces, real working hours/day, etc...)?

Anyway, you must remember, that:
Bigger voltage doesn't mean bigger capacity.

The traction and hydraulics motors for 80V usually have more power so they can climb up on bigger slopes, but paralelly it doesn't mean that 80V works works much longer than 48V.

80V are 20-40% more expensive than 48V.

Usually 80V are used for the trucks with bigger capacity (see your application).

In my opinion, you should consider the fact, that the fleet should taylored for all application aspects, not only the working time.