It's iffy. I drove with two Rovers on the back of the Explorer through heavy rain and flooded country roads once I got near the coast. Both bikes got SOAKED between the heavy rain and 2' flooded roadway splash a few times. I let the bikes dry off over night in the tent and half the next day.
I covered the handlebars and battery port on both bikes. While one bike had zero issues - the other bike sadly developed a ghost throttle and a mind of its own. I explained the situation to Rad and they didn't seem concerned about the wet weather. But I'm thinking the rain got somewhere it shouldn't have. Replaced the controller with same issue. Replaced the wire harness and the issue went away. Was it the cause of the rain?? Who knows.
However I think they are fairly waterproof but anything is possible. I'd cover them with at least a tarp. I went online to Amazon.com and ordered a 4-person instant tent and it stores both Rovers perfectly with a bit of room for extra gear bags. Not a bad deal for $89 shipped prime.
And we made some 3D battery port caps. This way when you remove the battery you can cover the connection port with a water tight cap. Naturally it will pool water in there and if you go to plug a battery in - you could have a boom boom show.