Insane is right, as we both know.
But it still doesn't excuse it. As you said, Billy would be doing most of this stuff (hopefully) if he lived on his own anyway, you're not really asking him to go above and beyond. If he doesn't like it there is an easily solution, he can move out, where he'll still have to do the same things for himself or live in a stye. And if he moves it's one less stresser on you and less mess.
I rarely have issues with Travis helping me with stuff and I'm not anywhere near as down physically as you are. He lives in the home, just because he's grown doesn't mean that's a free ride. Actually the opposite is true.
Of course that is what irritates us so much with how he treats you. There is just no excuse, even if you were perfectly well and able bodied. That boy (and I say boy cuz he acts like one) needs a severe attitude adjustment. I think if he attitude doesn't become pleasant and helpful fairly quickly I'd give him his move out date regardless of what's going on with the girlfriend. She's his problem, not yours.
But as for the girlfriend I've been wondering for a while now if she's scamming him. Aspies tend to Obsess sometimes instead of love......or maybe it would be better to put it as obsessive love? Which leaves them wide open for all sorts of scams and mistreatment by the wrong person. husband was that way with his 1st "love". It carried on for well over 20 some years even after marrying other people. I think I'd actually stroke if I knew the actual amount of money he's handed over to this woman over the years, not to mention other things he's done for her. She'd wag her finger and he'd react like a well trained dog, I swear. Doesn't surprise me because he does the same thing with me, only I don't treat him that way. Was she ever serious with husband? No. She used him whenever she grew tired of dealing with her husband or when she needed something. I think the woman must have died or something as she's not been an issue for about 5 yrs or so. I sort of wonder (and worry) that maybe Billy has gotten caught up in such a relationship with this girl. Travis has got a girl that he does the same thing with and has since his sophmore year in hs.....now she doesn't use him or anything, she's actually a pretty nice girl.....but he's so caught up with her that while she's going on and living her life he can't / won't consider anyone else. So I'm thinking it's a autism thing, at least with some of them. Know what I mean??