I have a stock 03 Dodge Ram 2500 , 4x4 5.9 cummins with an automatic transmission and from what I can gather , the general opinion seems to be these are not exactly the best auto transmissions in the market . I will be pulling a three horse slant goose neck with a 4' short wall dressing area that I am in the process of converting into a weekender. The empty weight of the trailer is 4800 lbs.

Should I be driving in over drive or shoul I turn the o/d off with the tow mode switch when pulling my trailer . I tried pulling in o/d on flat roads for short distance and the truck seemed to handle the load quite well without feeling any strain and when in tow mode it actually feels like it is over reving BUT the owners manual states to tow with tranmission in tow mode ( o/d off ). The last thing I need is a break down with a full load and fat bill for a re build tranny so any helpful advice from personal experience and knowledgeable individuals would be greatly appreciated.

You answered your own question.... Quote...  BUT the owners manual states to tow with tranmission in tow mode ( o/d off ).

I'm not going to contradict the statement of your owners manual, but..........

If you were to change your transmission oil to a synthetic product, you would lower your opperating oil temperatures and reduce the transmission's wear. Pulling three tons isn't hard for a modern HD diesel drivetrain. If you're using OD on the level and your oil temps are in the normal range, you're not hurting the transmission.

If you're pulling OD in the hills with a heavy load, that's a different environment. Proper cooling is the key to long lived components.

The early Dodge auto transmission (93 through 2002) were problematic.  The worst trouble was those trucks that had the pumps turned up for more power, and then towed heavy besides.       The 03 through 07.5 automatic was a much much better transmission.  And as mentioned before there are some things you can do to help  matters.    I recommend installing a extra capacity aluminum oil pan and running a good synthetic trans oil.   This reduces operating temps in the transmission quite a bit.    Having said this your truck should be able to haul the light load you described with no  problem.    I would recommend shifting out of overdrive on any hills, this is good common sense.   One thing that most Pickup truck owners don't know is that at best the auto transmissions available are not failure proof.   One of the most damaging things that commonly happens with the transmission is during backing up with a heavy trailer in difficult conditions.  This causes a lot of heat  and that heat is very detrimental to the life of the unit.    

Running the truck listed with the OD OFF while towing is a must!  The weak part of these trannys is the converters, in my opinion if you are doing alot of towing I would upgrade your converter and valve body in the tranny along with a deeper pan.  Check into www.cumminsforum.com for more details and you will find that SunCoast Converters is a great ugrade for these truck.

No need to run the truck with the OD off unless you just want to burn more fuel. I have an 04.5 truck, personal experience here. If you have it turned up the truck just be careful around 55 MPH on hills, you may slip the Torque converter, just back off. In stock form, just go. The only time you may want to go to OD off is in the mountains at slow speed or for EGT control. The 48re is a good tranny.  The weight you will be pulling is not a real concern. I am no where close to stock, so I upgraded my tranny at 38,000 miles with a new TC, Valvebody, etc. Go here to ask more questions.  www.dieseltruckresource.com

WOW, mine too from "personal experience"!!!

With owning a 2003, 2005, 2006, 2007 all with the same set up, 5.9 Cummins/auto tranny from the factory.  When in STOCK form all was great with od on with light loads, but when hooked to the horse trailer (3 horse Exiss then 4 horse Sooner) they shift in and out of OD all the time (unless we was on level ground) which is NO GOOD on the tranny at anytime. 

However after some upgrades with the engine and tranny I have no need for turning the od off at anytime.

Same with my 07.5 with the 6.7/auto, in stock form the tow/haul was needed pulling any trailer, now after some major upgrades in programing the only time I use the tow/haul mode is when I need the EB to engauge stronger.

Ahh, so you have the SO engine rated at 235 HP....

I have 4.10 gears and have never had an issue of shifting in and out of OD. If you have 3.73 gears then yes, this can be an issue. If you have any issues, it all can be fixed.... Lots of upgraded parts. I am partial to Goerend trannies. The main thing to remember is to keep the RPM's up, don't lug the truck. If you are on a steep grade it is easier on the tranny at 1900 RPMs and above than to be at 1600 or below.

It has a 5.9l TD (ETC) engine & 4SPD A/T 47 RE (DGP) transmission.