Depending on how steep the downgrade is, and how heavy your truck is, 3rd gear may or may not be able to hold your speed on on a hill.
I have the same transmission and on a low incline 5th gear can do the trick. However, I may have to drop down to 2nd gear on a really steep hill.
Additionally, I employ my engine brake. It automatically downshifts to 4th gear when I is turned on to low and I take my foot off of the accelerator. Once I place my foot back on the accelerator the system shifts back to 6th gear and stays there as long as I keep my foot on the pedal.
To try and answer your question on how to use the "manual" shift buttons here is what I do.
I check my RPM gage to make sure I am in the 1500 range. If so I will down shift one or two gears (depending) on how steep the hill looks and how heavy my load is). This will usually push the RPMs up to 1800 for 5th gear, and as high as 2250 for 4th gear.
If that still won't allow me to control the speed I will employ the engine brake (on low). This will usually push my RPMs up to 2250.
I then will use my brakes off and on to keep my speed within 5 mph of my target. Some people call this technic "stab braking".
As I near the bottom of the hill I kill the engine brake and then up shift one or two gears depending on my RPMs. I try to keep them between 1200 and 1600 as I up shift.
Each truck is different so I don't know what RPM range you should shot for. I am basing mine on the number of RPMs my truck is doing at 65 mph while on a level road.
If you watch your RPM gage while your transmission is up shifting and down shifting you should get a feel for the best ranges for your truck.
With a little practice you will get the hang of it in no time.
Hope this helps.
