Would depend on how your TV is acting. Will it hold OD while towing, or does it shift alot? Is the engine running free, or is it lugging / struggling? IF it's running free tow in OD. IF it's lugging, down shift untill it's runnig free. Lugging will use more fuel, and can cause the engine to begin to use oil.
You should be able to tell what gear the engine likes to tow in. Mine don't like OD. It luggs. It runs free in what is 3rd on my TV. While running in third the engine is way happier, running in it's powerband, burning all the fuel going to it, and I have much better throttle response, plus it does not gear down as much / so far on hills. Plus it actully gets better mpg, as the engine is not working so hard to hold a higher gear.
Just because you are in OD, does not mean you are getting better MPG, Or that it is not working harder.
Your results may vari.