Are you planning on carrying the dogs in the motorhome or having a trailer with the dogs? I'd think the latter would be handier overall. With a trailer, you might be able to use a class B rather than a class C. I'd think the driving would be less stressful, too, than with a number of dogs that may or may not appreciate each other or riding down the road in the same compartment.
If you're wanting to keep the length down due to concerns about driving, it's my experience/opinion that the length has not a lot of effect on how hard the RV is to drive on the road, although it does impact maneuverability in confined areas like gas stations etc. I have to pay much more attention to my width and my height than to my length. If there are other reasons, like how much room you have available for storage, that's a different matter altogether.