The idea of locking out 5th gear is to prevent lugging of the engine. You're carrying a lot of extra weight and the engine needs to be better centered in it's torque band to be able to handle this. The higher RPM will cost you a little bit in fuel economy but it's much less stress on the engine.

 

You're probably not going to need to put it in 4th for that sized trailer. I'd leave it in D unless you notice it shifting a lot between 4th and 5th. If that happens then put it in 4.

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I always towed my 2850 lb (dry) ~ 3500 lb loaded popup in D, no many hills around my area. Never had any issues.