I would not bring those bags to my room, especially in light of your disability. I highly recommend leaving them in the luggage rack on the lower level of the car. This will make it much easier to get them off the train. Generally the better sleeping car attendants will help with getting the luggage off the train. However, don't count on that.

You should have more than enough time at both stops to get everything on/off the train provided that you are ready, which is to say downstairs as the train arrives into Dallas.

All that said, Amtrak is very good about checked luggage. Not saying that they haven't lost a bag or two over the years, but it doesn't happen often.

As for where to stand, generally they will make an announcement at most stations regarding that. I can't say for sure that they do that at Austin, but it would surprise me if they didn't.