Sparky wears a STEP-IN Mesh EZ Wrap Bark Appeal Dog Harness
when outside and while we're traveling in our car or RV. Like you say, it's secure and comfy for him. We attach the harness to a lease attached to a seat belt buckle so that he has some freedom to move about and/or jump up in Sharon's lap as we're traveling. Here's one site that will give you more info on this and other harnesses: http://www.spoiledsweetpets.com/barkappeal.html
Sparky is a very respectful dog, both at home and on the road. We seldom have any issues with him and other campers' pets as we always walk him with his harness and lease. He does get over-excited when we happen to pass close to large dogs or large-breed puppies, so we need to remind him not to bark at them. A water spray bottle is a great help to get his attention.
When we leave him in the RV for an extended period of time, we leave a Kong filled with treats to keep him happy and occupied.
We hope you and your dog have many happy travels.
--Bruce and Sharon