It's a sign of:
Bad wheel bearing. Jack it it... is there any wiggle in the wheel/hub?
Overloaded (too much weight distribution!)
Alignment bad. Have you curbed the trailer, ever? It doesn't take much to push the axles out of spec.
Wheels not balanced. Have you ever had this done? It's time.
2013 Winnebago 2301BH-Red
2012 Ram 2500 Megacab HO CTD