Have you tried Green Mountain Ranch in Lytle Creek? It's located about 10 minute from Rancho Cucamonga off the 15frwy and Sierra. It has a ranch atmosphere, very cool. I would have gotten married there but at the time they changed owners and were no longer allowing outside catering so we ended up getting married in my in-laws backyard in Rancho Cucamonga. Good luck finding a venue! I hope this helps you out a little. I know the frustration because I was looking for the same type of venue for a rustic wedding and couldn't find much and it was very expensive. Green Mountain Ranch actually seemed to have very reasonable wedding packages.
There really are not any options in Rancho Cucamonga. I tried looking for the same type of venue. The only other place I could recommend would be the Christmas House in Rancho. I'm not sure if that is quite the style of venue you are looking for but you could check it out. There is a place in Corona off the 91 frwy and Gypsum Canyon Road called Star Ranch. It's up on the hill off the frwy and it's a true ranch, very country-western...kind of cool if that's what you are looking for. Good Luck!