What's the best way to maintain control on a steep downhill? Should I manually downshift the automatic transmission or does the 3500 do it automatically?

I have a 2008 Express motorhome with the cutaway chassis and a 6.0 Workhorse engine. We tow a 2017 Equinox and so set the trailer button.

 

Absolutely downshift.

Once you get used to it, you'll be doing it every stop.
You'll never replace brakes- remember how your car with a manual never needed brakes, and now that we've gotten slushbox lazy it's all the time?

PS, your owners manual will explain a bit more.

 

Also the selector doesn't put the transmission into 'Manual Mode' per se.
The transmission's governor will prevent you selecting a range that would result in engine overspeed.
IOW, I'm going 55MPH and select '1', the governor will not allow the transmission into one until the vehicle has slowed to a safe speed..

I don't know how it works if you select low gear and you are going down such an insane hill that the vehicle continues to increase speed- if the transmission would automatically upshift or not.
Some heavy duty trucks have a 'Engine Overspeed' warning light just for this occasion.

 

I can say for sure that the 2004 will just run away to a million miles an hour on a downgrade if you are in Overdrive. The transmission does nothing to prevent this, even in tow/haul mode, so you need to pick the gears with the stalk.
If you don't feel it downshifting then for sure do it manually or you will cook your brakes.

 

Your drivers manual, which comes with the car, has that information in it. If it doesn't - perhaps seek older manuals or call the manufacturer to complain they've cancel cultured essential driving information from the owner's manual. if you don't have one: get one.

If you do much towing you should be aware (the dealers do not always tell you), that you need A TRANSMISSION COOLING PACKAGE so your trans doesn't implode from all the towing. Alternates are: taking breaks to let things cool down. Installing a transmission fluid temp sensor (so you know what your dealing with).

The kits begin with a little in-line add-on, expand to a 2nd trans fluid radiator, end with a different model transmission that is for commercial duty.

Dealers often won't tell you. I know someone who blew a trans out that way - dealer never tried to sell him the packages.