We often travel with our dogs. We have a 4 dog trailer, which is essentially an aluminum shell with four snug secure doghouses, each with individual doors. There is a built in fan and ventilation system, and insulation, to keep them cool in moderately warm weather, and warm in the winter. We put in a very small electric heater for winter. In winter we fill the four boxes with straw so they can nestle in and keep warm. The dogs love their boxes to the extent that if I open one at home for cleaning, a dog immediately jumps in as if to say "let's go on an adventure". They are used to their little houses, and they sleep soundly in them.
This is definitely not an inexpensive solution!, but there are ideas here that might be useful to you:
1. We can stay anywhere: motel, hotel or somebody's house, and not worry about whether it is dog friendly.
2. The cooling fan runs off the car battery, so in most warm weather we can safely leave the dogs for a while in the trailer if we need to go somewhere they can't go, like a grocery store or a museum. In winter the insulated straw filled boxes keep them cozy down to at least -15C (they are shorthaired dogs), and we can plug in the heater if we're parked on a very cold night. You can't leave dogs in a car in warm weather or very cold weather.
3. If the dogs see something exciting outside the car, their excited paws can't do any damage. They can't steal your baguettes either.
4. If you need to, you can use all the luggage and passenger space in the car.
5. Don't have to endure dog farts while you are driving :-)
The upfront cost of ours was too high to consider this a money saving device, but it has saved us thousands of dollars in dog boarding over time. Very few people are set up to accommodate three big dogs as house guests, so without the trailer we would have to board them at Christmas, etc.
We still promote a dog up the the car if we want a little canine company some times during a trip.
We have friends who have modified a van to include a dog kennel section at the back, adding battery powered cooling fans and secure crates. That gives many of the same advantages.
For a more Mustachian take on this, you might consider building dog boxes into the back section of your car. I had this setup in my Element before we got the dog trailer. Make it so you can leave the back hatch of the vehicle wide open when you are parked, to give the dogs lots of air circulation. If you are expecting winter travel, insulate the boxes. If you're expecting summer travel, add fans to create continuous airflow.
People who haven't seen dogs in a setup like this before, are sometimes surprised at how much the dogs like their boxes. I was at first. I guess it is like always having your own home bed with you wherever you go. But I also think it must be because of some sort of den instinct; that the low roof and long shape pleases them. The dogs love to travel and have adventures, but there is something about the way they hop back in their boxes that makes you realize it gives them a real sense of security.