I'm going to kind of disagree about the motorhome manufacturer not taking into consideration the slide floor being a bit higher as when I look at the head of your mattress, it appears that it is higher than it should be. If you look at other "Air Dream" RV sofa beds, they all have the head of the bed lower than what yours appears. I've attached another image below so you can compare.
Therefore, I'm thinking that somehow, you have to lower the head of the bed somehow. Can you pull on it a bit more to see if it will lower the head of the bed?
Compare where the mattress is in relation to the back and arm rests of the sofa:
And see this video of a similar "Air Dream" bed and again, compare where the back of the sofa and arm rests are in relation to the mattress. The head of your mattress is definitely much higher: