I have 500 miles on my new truck and will be towing 500 miles in the next week. When I return, should I change the trans fluid? If so, how is it done- drain fluid, replace spin-on filter, save magnet ring, and add fluid? Is there a fluid fill area or do I simply funnel the fluid in through the dipstick tube?

I wouldn't worry about changing the fluid for 20K miles unless you want to put synthetic trans fluid in. I change the spin on filter every 10K miles (or every other oil change). Since your truck is new I'd change the spin on at 5K miles. There are a lot of opinions on this but that's how I did mine.

To add fluid put a funnel on the end of the dipstick tube and add tranny fluid as needed.

I would change the spin on after your trip and then every oil change or 10k or so. Pull the filter off, clean the mag, stick mag on new filter, fill new filter with ATF, replace. You wont lose enough to make a difference. I would check the fluid though to makesure GM didnt short change you. Also Change the fluid in the rear end and front end.

Check on page 2 of this thread:

http://dieselplace.com/forum/showthread.php?t=22403

This is for a flush & fill, usually used if you want to convert to synthetic.
Enjoy...