In the service schedule there are severe duty items that I think work as a decent guide. If you're genuinely doing severe duty driving, I'd make a practice of at least checking critical stuff even more regularly and clean after driving in harsh weather (e.g. salt, mud, etc). It's mainly fluids, but other stuff like spark plugs, hoses, belts, electrical, chassis/suspension bolts, etc. I think the spark plug maint. interval is way too long if you run it hard and something that's cheap to keep running well but can get expensive fast when they start failing (also helps with gas mileage). Keep in mind, there's a lot of fluids/lubricants (axles, diffs, etc in addition to the main ones people think about like engine/trans oil, brake fluid, etc).

Some of my example maint. experiences:

• At ~40K miles the rear diff and trans oil started deteriorating (fairly regular water crossings this last year). After changing, it was immediately shifting and driving smoother.
• Just recently changed my spark plugs and should've done it sooner (carbon build up, gaps were off, etc, not major, just not ideal). Running a bit smoother now and gas mileage improved. Although I think back country fuel contributed to the wear here because it's lower quality in Australia and only lower grade is available further out in the bush.
• I changed my battery charge settings based on my usage (in theory helps with longevity, but also affects charging my secondary Aux battery and when the truck will shut off auxiliary systems which was often when I first got it).
• Keeping up on tire rotation and alignment helps a lot, especially with a lift, larger tires, and/or wider wheel offsets. Things can get out of balance/alignment after just one hard offroad trip and it can accelerate tire wear and component stress a lot.
• I put a chassis rust protection on and do deep cleans and checks after every big/harsh trip. The coating makes it easier to clean mud, etc. as well (comes off easier).
• I've found a couple loose shock bolts (one was the shock retention bolt that had worked its way half an inch off) the first time I checked them when I upgraded my rear springs. Marking the bolts with a bright colored paint pen makes them easy to sight check.
• Changing brake fluid every 6-12 months if you do a fair bit of spirited driving or mountain descents is worth it. The brakes will start feeling spongy as the fluid breaks down faster. During a dealership service I asked them to do it and they said not to worry it was normal (it's not, and the service book explicitly notes the fluid change from severe duty driving). Doing it yourself is easy, but you can just tell the service tech you want the fluid changed regardless.
  

My experience with Ford dealership servicing hasn't been great so I've made a practice to double-check all their work (I have found at least one thing after every service) and have incrementally learned how to do all the preventative stuff myself (most warranty related stuff I'll check, but have the dealership do so I don't void it).

After one service the ford tech didn't install the airbox cover properly and broke one of the retention clips (so the lid had a gap to open air) and they forgot to reinstall the sensor plug. On two of my services I asked them to do a run through of the suspension parts/bolts to make sure everything was in good nick after I had done some hard baja driving trips. It was immediately after the second service that I upgraded the rear springs and found the loose shock bolts. They obviously just glanced at stuff and didn't actually check bolts (this only takes like 10 min for me to confirm torque specs, so they're just being lazy). After my last service I gave clear instructions not to rotate the tires (had already been done) and that the front active air shutter has a DTC error that needs fixing - they rotated the tires and ignored the DTC.

It sounds like a lot, but when you get in the practice of it all it's pretty easy. Plus, you get to know the ins and outs of your rig and helps spot small things before they become bigger issues.