Some of us believe in the "change a little bit more often" approach. If you have to change the trans. fluid every 30,000 miles, you can get it flushed at a dealer or shop, or you can change half of it every 6,000 miles. Even though you only get half of the fluid the first time, you will get half new and half old the next time, and three-fourths new and one-fourth old the next time, and so on. Even though you get less of the old fluid each time you change it, you will still get a little bit more of that old fluid out. The percentage of old fluid changed this way is probably the same or better than having it flushed once every change interval.
The advantages are many.
One, no one else is messing with your truck. You don't have to go to a shop.
Two, the cost is spread out over time.
Three, your fluid is more consistently fresher throughout the life of the transmission.
Four, it's easy to go ahead and change the filter when the pan is already drained.
Just change the filter on the regular change interval, and change the fluid more often. Just remember to add a drain plug to your pan when you are changing the filter, and it will be a lot easier. If you are not mechanically inclined, get a shop to add a drain plug to your transmission pan when you have it in for service. If they object, take it somewhere else.