One of our dogs had to visit the vet in Jan of 2008 and long story short, he's still alive, but one of the problems we were having was that we couldn't keep food in him. He was constantly throwing everything up. Well, the vet perscribed Famotidine 20mg that was good for keeping the food down and also for long trips in the truck. Famotidine is also known as pepsid (http://en.wikipedia.org/wiki/Famotidine
). Perhaps that might help your situation with the 3 p's.
1 pill 30 minutes prior to driving (for us it was prior to meals, but it works both ways.) which was great. Our dog's can now go to places like Granby and Grand Lake without leaving a mess in their pod or backseat.
Where we've been:
2000 Rockwood Roo M-18 Hybrid
2006 Dodge Ram 2500 Auto 5.9 Diesel
Gone but not forgotten -1986 Rockwood 1003 PUP