No, you have to pull out the generator to change the oil. It's a silly design, yes. I heard of someone that has a hole cut in their floorboard so they can drain the oil without removing the generator.
On the other hand, the generator isn't one of the really heavy ones - it's about 113 lbs, if I remember correctly. (My generator currently isn't working, and my dad and I are going to try and fix it during an upcoming trip in a few weeks, and we expect to have to pull it out of the camper to do that. I'll let you know how it goes.)
You may find the Yahoo Groups for 190 owners helpful:
http://autos.groups.yahoo.com/group/Airstream190/
I don't know where to get parts for the 190, but I'm interested in finding out, too, since I bought a 1991 B van earlier this year. Enjoy your new Airstream!
--RJ