Seen this discussion a few times over the years, most people do not add grease to the trailer ball, a lot of people do, argument follows the line of grease is messy, dry will take forever to wear out the trailer ball and/or the hitch.
I agree, the dealer was speaking incorrectly, even if there was grease there at one time, it doesn't last forever.
If you are willing to put up with the grease, lube it heavy, otherwise for infrequent towing (normal camping usage) you probably won't harm anything
Personally, I like to spray it often with WD-40, it's not messy, it provides a light lubrication and makes me feel better.
2011 Dodge Ram 2500 Cummins
2012 Starcraft Autumn Ridge 297 BHS