So you're saying I shouldn't leave it in tow mode the whole drive?

Don't listen to this - any time you have more than, say, 4000 lbs on tow you should be in tow/haul. This applies to city/highway flat/hills/mountains.

Tow/haul alters the shift points to be higher in the RPM bands. As someone else noted, you want your engine to be a bit higher in RPM for a given speed when it is pulling a load. This reduces the heat load on the engine, reduces the torque forces on the crankshaft, and most importantly the transmission, lets the transmission run a little cooler, etc.

I don't have a 5.0 but from looking at them I would expect to run over 2500 rpm almost all the time when pulling, up to about 3500 on the flat highway. Lower gear and higher RPM any time the wind or grade is tougher. Don't try to get it to run in the RPM bands that it uses when empty - that will hurt it as much as over-revving.

I wouldn't expect it to run much over about 4100-4200 for any significant amount of time. If you're up in the upper band, > 4500, consider just backing off the throttle and letting it work at about 4200. If it seems like it's lugging (meaning you push the throttle pedal and it can't accellerate) you're asking too much of it for the gear it's in. Either downshift or back off the speed to let it get back in the sweet spot.

Mostly the drivetrain controllers know what to do. The only thing to really be aware of, if it's constantly shifting up then down, repeatedly, then probably use the selectshift to lock out whichever high gear it's unable to stay in.