Not sure on the Montana's, but many many cars don't even have a gasket from the manufacturer. They just have RTV and the Felpro gasket has been shown to do more harm than good when it comes to the oil pan.
Another issue is what kind of sealant did you use? Did you follow the directions to the tee? Many of the sealants you can't tighten right away or else it will definitely leak. Many you have to finger tighten the bolts and let it set for a period of time before you actually tighten to spec.