If you don't already have them, get yourself some clear turbo lube hub covers (go to:
http://www.tiedown.com/amarinedls.html for more details). I check them everytime I get out of the vehicle when trailering to make sure they have enough oil in them. It's also an easy way to see the color of the oil. Don't forget to feel their temps too, like someone already mentioned.
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2008 H220 Frenzy - 5.7L VP SX Drive
2013 Tahoe LTZ Tow Vehicle (admiral's ride)
2011 Ford F150 XLT Ecoboost Tow Vehicle