The platform and mattress in my trailer was a RV short queen. The "mattress" is really a 4 in thick cushion. I left the cushion in place and bought a full size queen mattress. I laid the full size queen mattress right on top of the existing cushion. There is still enough room between the mattress and the wall at the foot of the bed to shimmy around to get to the other side. With or without the cushion in place there was never really enough room to sit at the foot of the bed while putting on shoes and socks. The full size mattress does hang over the foot of the bed, but has never been a problem, since there's not enough room to sit on the end of the bed anyway. I have never done any mod to the platform although it does need some gas Pistons added to help lift the mattress and platform.