Recommended Maintenance

Transmission fluid/filter, axles, transfer case, coolant, brake fluid and power steering fluid. And spark plugs.
I'd do that all now and then do nothing except oil changes for the next 75k and 8 years. Except maybe the trans again. Idk what the interval is on your truck.

Or you could read the owners manual and go by that.

Great advice from Grit dog.
Don't forget the air filter and serpentine belt and I'd change the belt tensioner pulley while your at it.

Serp belt is good to have a spare, I think, but since those have been standard issue now for the last what 25 years, I have yet to see a serpentine belt break unless it was destroyed due to failure of another component. I carry a spare in the Dodge truck just because I've had to replace the belt due to squeaking. But anymore, it's not on my maint list.
At 75k miles I wouldn't expect problems with pretty much anything. We have a fleet of older GM gas pickemups. All 6.0s and all abused and never really any engine or drivetrain issues. Brakes, unit hearings, u joints that kind of stuff on 100-175k mile trucks.

Replace all fluids including brake and PS.

If you able to do work yourself all you need to do with Allison transmission is change spin on filter and drain and replace transmission fluid with Transynd. Do Not let anyone power flush that transmission. The transmission pan doesn’t need to to be removed to replace internal filter...leave it alone

Allison is adamant regarding no power flushing of their transmission as well as no need to drop pan and replace internal filter unless over hauling

Also absolutely no reason to replace the serpentine belt idler pulley... waste of money to do this unless there is an issue... Serpentine belt should not need replacing this early.

Be sure to replace radiator antifreeze with ONLY Dexcool unless you want clogged radiator and heater core issues..

^All good advice, exept his truck doesn't have an Alli.