It was probably originally lubed with a Polyurea base conventional wheel bearing grease so just about anything else you're likely to get your hands on would be compatible.
That said, there are incompatibilities between greases (base-soap thickener materials) and some that should never be mixed. I'm kind of anal about the grease in my trailer's bearings and have thankfully never smoked a bearing - including watercraft/boat trailers which are as unforgiving to proper maintenance as heavy loads. Consider the intended use. If you're prone to drag that trailer through water, I would clean out all the old grease and use Marine grade wheel bearing grease now and forever.
http://www.etrailer.com/faq-grease.aspx