Welcome to the forum. I do not have the tilt a bed system nor am I familiar with it, but if it is the Lippert system (which Thor typically uses for almost everything) there is a link attached to open the install manual info for the bed tilt mechanism. In the install instructions it says the mattress is held with a strap but the picture does not show where or what that strap looks like. I would assume it would be at the foot of the mattress otherwise you would be laying on it.....or perhaps it goes where the mattress actually deforms and bends up towards the head end. I am sure someone else with the system will respond to this so we can all learn.
https://www.lci1.com/bed-tilt-system
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