Many dealers are cautious about test drives - they get more than a few tire kickers and test driving an RV is an expensive risk for them. I suspect if you go there and appear serious, i.e. it's the "point of sale", they will accommodate you. If you just want to experiment with various sizes and styles, expect some reluctance from some, if not most, dealers.
Unlike the others here, I don't think a test drive is all that important for an RV. The house part is of much greater importance than the vehicle part and as a newbie, you are going to have a learning curve no matter what. They are noisier than a car, accelerate & brake much more slowly, and also wider and longer. A Class B or C will seem more familiar initially, but you would quickly adapt to the different view/seat position of an A.
All that said, if one dealer won't accommodate your needs, move on. There are lots of fish in the ocean...