It's true you want to avoid activity that puts tension on your incision lines after surgery. The more tension you place on your incisions, the more likely you are to have a widened scar or wound healing problems. Some tension is unavoidable, but you will need to make some changes in the initial postoperative period. You will have drains in place, and will have dressings or garments that you will need help with. You are correct in recognizing it will be difficult to go to the bathroom initially, and you will need help with some basic activities early on.
I typically have my patients go to either an aftercare facility or hospital for the first night or two. Some patients prefer to stay longer, especially if they are in a situation where they do not have enough support at home, or if they have too many demands on them at home to allow time for peaceful recovery. (This is often the case when the patient is the primary caretaker of everyone else at home!) Afterwards, I would recommend temporarily staying with a family member or friend so you are not tackling 3 flights of stairs right after surgery.
Keep in mind you can also have surgery in two stages, with the circumferential body lift (or belt lipectomy) in one surgery and the thigh lift in a separate surgery. How many areas you include in one operation does affect your recovery process and recovery time.
I would recommend consulting with a board certified plastic surgeon to see what you are a good candidate for, come up with a plan that fits your needs and life demands, and go from there.