Unfortunately, I do know the position you are in. I had to have sex for five years in all kinds of places...behind a church, in a ditch, in a car, in a bathroom, on a playground, in the woods, at a bus depot, in a ravine...yeah, a lot of places.
It kind of sucks, because you are like "I'm a adult, dammit." I'm 21 and for financial reasons I still have to live with my family. If my boyfriend didn't have an apartment, we'd have nowhere private to have sex, because my parents forbid sex in their house (though I have definitely broken that rule a few times). I have no problem going back to doing it outdoors (I'm kind of an exhibitionist, anyway), but that's an issue in your relationship.
I do not understand why she won't do it in the car as long as it is on the outskirts of town and secluded. I mean, there are some outdoor places that are almost as safe as indoor ones. I discovered a couple of them in my area, and most places have some. But if she won't, I guess your best bet is a lock on your door. I don't know what you could tell her that would make things easier, but no one is actually going to "catch" you if the door is locked. If they knock, so what? Tell them you simply don't want to be bothered right now; you got sick of all the people in your apartment or whatever and you just wanted some peace and quiet. And if they know you are having sex, oh well. They already know you are doing it, they just don't make it obvious that they know. I mean, no man takes his girlfriend to a summer house for a PG-13 weekend getaway.
I know you can't really explain all this to your girlfriend. I don't even know if it helps you out; after all, I don't know the girl. But my point is, people know you are having sex. And yeah, getting caught really sucks. It's really scary, and it's really embarrassing, especially if you get caught by people with legal authority (had this happen once...thank God we hadn't actually gotten to the sex part yet). But while it's often mortifying at first, it's often something you can laugh about later. Maybe your girlfriend doesn't feel that way, but know that a) a lot of people get caught and b) all of them live through it.
Sorry for this ramble...I am running on very little sleep. I hope I got my point across. If you need me to clarify anything, just ask.