I actually threw my back out about 2 weeks ago, the morning i was supposed to check in on a hotel assignment. As someone who has pretended to have a bad back for years, it gave me a lot of insight into how to make that convincing.
As Wales suggested...just walk slowly. I think I was probably wincing when I slowly stepped out of the car and the bellman ran over grab my roller bag for me. It also gave me reason to call down for ice, ask how late the jacuzzi was open, inquire about the nearest place to get Advil, eat all my meals at the hotel, etc.
That said, when I am driving to a hotel evaluation, I have a file box in my office that's full of papers. I have labeled something like "receipts and invoices 2016". It's probably 30 lbs and there's no way it's getting to the room without a bell cart, so I toss it in the back of the car in case I need to ask for help. It also helps with the cover story of why I am checking into a local hotel; I moved recently and am back to see my tax accountant.
Though I didn't need it for the last hotel, I could not even get the box out of my car for a week afterward! (FYI; I threw my back back out picking up a sock, not loading the heavy box into my car. Now I really feel old!)