Best two options I got on my 19 were the solar and the outside shower. We had the shower installed on the passenger side, and use it for doing dishes outside.
Another thing I chose that I like was the large fridge.
But these are, of course, personal choices. Solar 'cause we tend to get sites without power. Shower outside 'cause doing dishes sucks, and doing dishes in a small sink with no counter space sucks even more. Large fridge 'cause, well, the wife.
2013 Escape 19 ("The Dog House") , 2012 Nissan Frontier