I know this is an old thread, but figured I'd post here rather than a new thread, because through my research this is always one of the first threads to come up.
After calling and speaking to many rangers in the NP and NF around the Yellowstone parks, in anticipation for my Yellowstone/ Grand Tetons area trip the recommendation has been:
Hardside "lockable vehicle" are okay and legal to store food in fridge, and "airtight" containers, even if you are sleeping in the truck. Just make some effort to conceal the containers, when in the trucks or leaving for a hike etc.. Also, cleanup and odor reduction is key in general.
Softsided vehicles/ pop tents, etc.you need a bear bag, bear canister, bear locker, etc., and store them outside the truck using normal precautions/ standards
No warranty given for this information, and if I never post again in a few weeks, disregard :Wow1: