Hi Fordbossguy,
I wouldn't use the 5w/40 if it were me. Your 6.2 does have a water to oil cooler that should help keep the oil temps in check so I think you'll be fine running a 5w/20 that meets Ford's specs and using a synthetic will hopefully keep the VII to a minimum to help with shear considering that engine will likely be spinning higher RPM's for longer durations than the average commuter / weekend warrior truck would. I don't think I'd try to extend the OCI's with that load, Ford says 5000 miles with regular towing and that's where I would start. I don't think the 6.2 ever got the IOLM so if your truck is like mine it's just a mileage counter (I have a 2011 F-250 with the 6.2) and doesn't account for load, speed, etc.
I'm interested in the performance of your truck with the weights you tow, as it seems you'd be a bit over GCVWR even with 4.30 gears pulling 19k. I'm assuming you're using a 5th wheel car trailer?
I am impressed with how well my 6.2 does, it's got 96k miles and it pulls great. I pull a John Deere 110 TLB on a twin axle flatbed trailer hooked to the factory receiver with a weight distribution hitch, I'd estimate my combination is around 17 to 17.5k with some tools in the bed, and you're pulling more that that with your truck, wow! For my truck I use a name brand conventional changed at 7500-8000 miles with a MC FL-820S filter, because even though I tow something nearly every day it only accounts for about 30% of my total mileage.
As for the brake and power steering fluid, changing them both every two or three years won't hurt depending on how hard you work those systems. If your putting 100k miles a year on a truck over flat interstate you may not need to worry about those fluids much especially if you've got the gain boosted up on your trailer brakes. But if you're putting 20-30k a year on with your weight maxed out through city streets you might want to change them more often. BTW, Ford does recommend changing the spark plugs at 60k if you tow regularly, I might do the transmission fluid at the same time.
Good luck to you.