I would say, more than likely, a good drink before a car ride would cause water to slosh around in the snake and come back up with mucus to make it bubbly. If it is RI, it sounds mild, otherwise it would be breathing audibly, opening its mouth and puffing out the throat. I wouldn't bother with a vet at this stage, keep an eye on it and if any symptoms persist, then have a vet deal with it.
Raising the temperature as you have done, will, in many cases, fix a respiratory infection anyway. Antibiotics are not always called for, and it is far better for the animal if you can avoid using them.
The most certain way of telling whether it is RI is to offer food - if it eats, you have nothing to worry about.
Jamie