Most of our RVing was done with a large Australian Shepherd and a Cattle Dog - both are high-energy dogs. If we were going to do something that they had to remain behind, a brisk walk was sufficient to have them snoozing while we were gone.
The biggest advantage? Protection. We had a sticker on the door of the RV that said "dogs on board". I figured when we were parked for dinner at a restaurant or whatever, any thieves who might be eyeing our RV would go elsewhere if they knew dogs were inside. And if they were bold enough to try the door handle, they would be met with some serious growls. Granted, a teacup poodle is also protection - but a GSD growl is probably more convincing :-)
The disadvantages? At National Parks, you are limited to the paved areas of the park. Hubby and I aren't hikers, so this is no problem for us. A large dog sprawled in a small RV makes moving around difficult. When you arrive at your site, instead of sitting back and sipping a beverage, you will be walking a dog. And ANY dog rapidly learns to LOVE camping - as you start to pack for a trip, you will be listening to excited whining until you actually allow them entrance into the RV......LOL!
On the other hand, a dog encourages you to be more active, and you generally meet more people. I think my life is richer and healthier, thanks to my dogs