I have seen custom skid bars welded to the frame; have seen many types of castors, solid/pivot/large/small; and also a rotating roller that was about 3' long/wide. Lots of ways to handle this, just be careful you don't put too much stress on the frame when the castor/roller/skid takes weight off the wheels as you go over the dip. DON'T mount anything under the bumper...
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired