This is my take on it
Firstly I am in a very similar position to Joe and have the
SW 18" goto Dob under excellent dark, steady and transparent skies and it is simply fantastic viewing!! Were I not permanent and was keen on a dob for dark sky viewing that required pack up and significant travel and was keen to make the most of my trips out to great skies, I would snap up the biggest scope that would fit in my car and one of these SW20 goto dobs would be perfect
. The way to look at it is and certainly if you have received the finance go ahead (
) why go to all that trouble to travel to a dark sky, only to get there and set up a piddly 10 or 12"..?just becasue it takes 10min less time to setup, and three steps less to view the eyepiece!? (although I do have
one of those too 
) the views will be well worth the slightly extra setup and packup time! I've looked through a few large dobs over the years, under some of the best skies on the planet and a 20" class dob under a truly dark sky is truly amazing, you wont regret it!
So, if you can indeed fit it in your car and that's your main method of use, go for it I say, the views will be worth it!
Mike