Its a question I'm asked a lot, and I've tested tons of options without ever finding something thats going to be paint safe and easy to use around your truck.
I had the best luck with the 'flow thru' style RV boars hair brushes, where the brush is continually flushed with clean water, but the drawback was it took the soap with it as well so you ended up with no lubrication/soap on the panel, and thus swirl marks. Plus it was extremely expensive for a quality one.
Personally, I climb in the bed to do the back half of the cab and the forward half I use a small step ladder with good grip so wet shoes don't slip for the front half. Sucks having to climb up on something to wash - but part of the deal when detailing/washing a big vehicle.
Look on the bright side - its REALLY easy to clean the fender liners versus something low