OP, clearly, this should have been discussed and layed out before the inlaws were invited and the room was booked.
I'll have to be the dissenter here....
Of course, the solution would be different for different people... but, if I am planning it and I am paying for it, then I would get the bed.
Personally, I would be hard pressed to agree to travel without adequate accomodations... And I certainly would never accept free accomodations and then actually take the bed while the ones who were paying had to take the couch. (but, we have seen enough threads about people with 'entitlement' issues.)
If 'respect' and 'age' warrants that the inlaws get their own room with a comfortable bed, either get a larger unit, or an extra studio, or see if they could afford to chip in on the accomodations (I don't think the OP mentioned their financial situation.) I know that I would be thrilled just to be invited to travel together, and would never expect to have premium accommodations all paid for.
OP, unless you are ready to 'gift' them this trip, by reserving a larger unit or another studio..... And, if you can't afford to offer them private accomodations or a real bed, then maybe they shouldn't have been invited to come along on the trip, for free, like a gift.
One should be able to afford what they give.
And one shouldn't have hard feelings because they had to 'sacrifice' (sleep on the darn sofa-bed) to give.
Personally, if I don't have my own bed.... I don't go....
I almost ended up in a similar situation.... my sister wanted to plan something for her birthday, and we were planning to go to the mountains to see the fall colors. Because of October high season, the room rates are sky high!!! And, when given the choice for either the two of us to go one weekend while my husband was away, or for her to come with my husband, my son, and myself on the next weekend, she chose the 'family' choice.... But just assumed that we would make room to offer her accommodations... that would put all of us in one room, short a bed.... I do not think that she was offering to help pay for the room, or get another room...