If the center of your hub has a plastic cap (like BarryD said) or a rubber cap that can be removed, chances are you have a grease zerk under the cap. If you do, life just got easier.
Jack up the wheel you want to grease til it's off the ground. Attach your grease gun to the zerk. Give the wheel a spin and while its spinning give it one full pump on the grease gun. I do this at the beginning of the season.
I use a good synthetic grease that is the same as what's rated for a boat trailer.
A rainy day campin' is better than a sunny day workin'.
'13 Windjammer 3025W
'14 Ram 3500 w/6.7 Cummins
No dogs. No Cats. No kids. Just us.