Holding on whilst running would be difficult. But there are steps at the back.
Robs crew have said that he would often run behind, and allow the RV to drive on at running pace during the night, and would then signal them with his flashlight if he needed them to stop. I could see a situation where he could run behind and then climb aboard the ladder without the driver even noticing. Occasionally jumping down, letting the RV get a little way down the road and then signalling them to slow down. I'd think that at least one of the crew would have to be in on it, but not necessarily all of them.
There was a fishing line attached to the RV and Rob would hook it onto his belt buckle.