I think that if attaching an antenna to the ladder worked well, we'd see more of it. I for one have never seen it.
A groundplane antenna needs a certain minimum metal surface area to work properly. The ladder may not offer enough area.
On my previous gas rig I bought a high quality non-groundplane antenna. We mounted it at the front belowthe roof line on the side panel. The coaxial type cable comes cut to a specific length and you mustn't shorten it. We ran the excess cable behind the cabinets mounted to the inside of the front cap.
My reception was outstanding and my caravan companions reported that they heard me well at a couple of miles distance.
Fulltimers! - Currently in Gainesville, FL - working as Webmaster & Special Projects Manager