Don't make a mountain out of a molehill. Turn the O/D button ON and leave it alone. Most automatic transmissions do a better job of shifting than most drivers, anyway. If you're advanced enough to be pulling a trailer, you should already know what you're doing and shouldn't have to ask anyone's...
Check your oil frequently (figure it out) until you determine whether the engine uses any. If it uses oil, check it more frequently. If it DOESN'T use oil, check it less frequently. Do what you're comfortable with.