When I first bought my boat I was given the advice to only trailer it in 3rd gear and that is what I always do. Is this what everyone else does even on long flat highways? Can someone explain what the problem is when towing in Drive?

And another thing, why is it so bad to use Cruise when trailering?

What is your tow vehicle?

Yeah vehicle would help.

Towing in cruise has some different opinions. It won't hurt anything from my understanding, but your tow vehicle will leave its gonads on the asphalt trying to maintain speed going up a hill. By allowing the vehicle to slow while going up a hill it is easier on the vehicle and better for fuel mileage.
Using cruise on the level though will help you maintain a constant speed.
But using cruise allows you to become a bit lacs with attention which isn't good while towing.
Just some things to think about.

I don't used O/D to keep the shifting to a minimum. I use cruise, but only in the boonies. In traffic I think it's much quicker (safer) to see a situation ahead and adjust your speed with foot pressure. In cruise I think most people wait a longer time to adjust speed, hoping the situation will go away. Getting to close to vehicals ahead.

It's very annoying to be in the left lane behind somebody whos in cruise and "passing" traffic in the right lane by 1 mile an hour. If your in the left lane GET MOVING.

Tow vehicle is a Chevy Trailblazer and adequately tows my boat.

I think that were asking more from a point of newer versus older. Just take it out of OD if it shifts in out OD. That creates too much heat and heat is bad for the transmission. Cruise is more of a safety issue in my opinion. It takes longer to react to something while you have cruise on, versus the vehicle slowing down as soon as you take your foot of the accelerator.

Around town and in hills definately out of OD, on the highway try it and see.

I use OD and cruise control all the time. If the RPM drops below around 2200, the engine is lugging and heating up so I take it out of OD and use 3rd. Your results WILL vary as this is particular to a diesel engine that is oversized compared to the load that I am towing.

On the flats I put in in O/D, in hills and around town and traffic, I use the 'tow/haul' option.

Keeping it in 3rd all the time wastes gas IMHO. You need the extra power in hilly areas, but if you're on the flats, put her in D with O/D on. If she starts hunting for lower gear, drop her down.
And I concur with cruise control... not good for attentiveness, or quick actions.

When I towed my boat with a Suburban, tow-haul mode really wouldn't engage o/d until over 60mpg. I'd tow in o/d as long as it wasn't constantly shifting back and forth. With our travel trailer o/d isn't an option. The Suburban wouldn't pull it in o/d even on flat ground.

My Expedition has a 6speed trans. I can't select 5th gear. If I use the o/d lockout it goes uses 4th which keeps the rpms near 3k @ 60mph. I just to in o/d and it tows my boat in 6th fairly well. I haven't had an issue of to much shifting between 4th-6th, but if I do, I'll certainly lock the 2 o/d gears out.

I use cruise on the highway. In my Suburban the cruise was slow to react. The CC would loose 3-5 mph up a hill then kick down to second to catch back up, so I didn't use cruise in the hills.

Likewise, in traffic and hilly area's I just turn off OD. Shift to 3rd only when needed while going up steep grade. I let the RPMs dictate what input is needed from me.
On long flat roads, I use OD and may use CC for a stretch.

Just keep an eye on the tach and the terrain.

You should not be trying to tow your boat up a hill at 2000 rpms.

On the other hand, going downhill at 3000 rpms is a waste of gas unless you need the engine braking.

I don't use cruise control while towing, it is too unpredictable.

Whoever told you that was trying to help you avoid the transmission 'hunting' for the correct gear, which puts a lot of wear on the tranny if it is constantly shifting up and down.

You will be able to feel that if you are paying attention, and in such case go ahead and stay in 3rd. But other than that, you should be ok in Drive.

I don't know, troop. The cruise control in my truck does the job for long hauls on the interstate, but there are times when it needs to be overridden. Even with the cruise control on, I still activelly participate in driving.

Does your Trailblazer have a tow/haul switch or an OD switch? If so then just use that to lock out The overdrive and just use third gear for steeper hills.

Alot depends on your rear end gearing as well. with 373 gears you will have more towing power than with 355 gears and can use the OD on flat ground.

I'm pretty sure it has the OD switch.

TEG:

In your situation, I think it's best to keep it in 3rd. The 4.2L I6 engine is a fantastic power engine, but it's rev happy. The fuel burn between O/D and 3rd is at best 1mpg difference. Realistically, you're only looking at a 0.3/0.4mpg savings. There are a lot of risks involved. You can try the above mentioned put in O/D methods once you're up to speed...but if the rpm's jump at all (200rpm is NOT acceptable) at any point to maintain speed, I would pull it into 3rd. Your engine/transmission are less stressed in 3rd than O/D in this loaded situation. The combination runs cooler in 3rd than O/D. Heat is the killer.