Cut onions in half lengthwise, slice in 1/4 inch slices, halve the slices and separate into rings. Trim all possible fat from roast and sprinkle with garlic salt and seasoned pepper. Place half the onions in crockpot, put in roast and top with rest of onions. Pour 1 cup of soda over all, cover and cook on LOW for 10 to 12 hours, or overnight until pork can be easily shredded with two forks.
Remove pork from pot, shred in a bowl, removing all possible fat. Remove onions with slotted spoon and add to bowl. Discard juice in pot. Return onions and pork to pot, add BBQ sauce to all and mix. Continue to cook on LOW for 5 to 6 hours, adding more BBQ sauce as needed.
Serve as sandwiches in buns. Leftovers may be cooled, made into sandwiches, wrapped in heavy plastic wrap and frozen individually for later use. They keep well and may be unwrapped, rewrapped in paper towels and heated in a microwave at 50% for a few minutes.