Protecting bikes from elements when traveling suspended on hitch bike rack I'll be traveling this summer (1000km each way, with one stop in between) and would like to take our bikes (4) along with us. 

I have a Thule Apex installed in a 1.25" hitch that will be host to 2 mountain bikes (29er, 27.5), 1 woman's hybrid and 1 kid's mountain bike (20"). I have storage at destination.

How do I limit the impacts of the elements (rain, road debris, etc) on our bikes, most particularly the disc brakes and drive train, when traveling?

Edit #1


Consider the encountered wind (ie. highway vs. storage) as reviews seem to indicate that certain products may not withstand various degrees of wind. 
Using RV in the keywords also tends to help
There are several companies that make covers for trailer hitched bikes.  It's essentially a giant bag that's designed to cinch down over your bikes and rack. For a long trip like you are suggesting, it's probably well worth getting one.  I have seen bikes after such road trips on a rear rack coated in horrible, horrible road slime.