Hi lilflower,
How long are your in-laws going to be there?
If it is a long while, then it is important that you convince your hubby that you need a mattress to ease your back pain...
My advise would be to go to a doctor and get his opinion as well...maybe that would help to convince your hubby....
The reason I'm saying this is because I suffer like you...without a proper mattress, my back goes for a toss...unlike popular perception, sleeping on a hard floor is not always the best thing...it depends on person to person...I always stretch a muscle when I sleep on a comforter or a thin mattress or a bare carpet floor...end up with a headache also...
So do what is best for you...after all it is your body that will suffer...
take care,
Aarushi