Hi Beth:
I agree with the others NOT to carry gas inside the RV. But, I want to add, having traveled to Yellowstone and Grand Teton NPs about 6 times from different directions, that you should not have trouble getting gas when you need it. You can get gas at reasonable prices in Jackson WY, just south of GTNP, and there are several gas and repair stations within YNP. Just plan ahead and fuel up when you get the opportunity to do so. Note that there are some very small towns in Wyoming that are just a spot on the map and have no gas service. In fact, there are large areas in Wyo. & YNP that don't have cell phone service, so don't push your luck.
Bob