I also have a W-20 chassis and had a problem getting the radiator cap off. Like you I could turn it about on quarter but not enough to come off. The cap has to be depressed for the last inch of turn. I couldn't get leverage on it because of air intake and overflow bottle postion. I finally just took the overflow bottle out of it's rack and was able to get enough pushing leverage to remove the cap.
2005 Winnebago Adventurer 33RV | 2004 WH W20 Chassis | 2010 Honda CR-V Toad
USAF Ret June 1990 | 42 years continuous Active Duty
49 States | 10 Provinces | 50 years RVing