I had the same problem on an older Winnie. I found out that there was no ground strap from the frame to the engine! I found out when I had starting problems and (after replacing several starters) found that using a jumer cable directly to the body of the starter and frame, the coach started with gusto! I looked all over the coach for a busted ground cable and found there was none. I installed one from the frame to the starter and the problems with the wipers went away.
2004 Itasca Sunrise 36', Workhorse
2003 Suzuki XL7