As far as replacing the carpet with vinyl...no problem. When we got the current 5er we found the carpet in bedroom had been torn by the slide moving in. (I won't go into the reasons why, but we got the cause cured.) We cut and removed the damaged area and replaced it with good quality self adhesive vinyl squares. They were easy to cut to shape and reposition as necessary.
With regard to the hatch you want to make...so much depends on the position and amount of access you need. The first thing that comes to mind for me is to cut out the size you need and mount it back in with a piano hinge. You'd have to mount something around the edges of the hole you've cut to provide adequate support for when the area may be subjected to someone walking on it. That support could be wood or metal (I'd prefer metal) and secured to the floor with recessed carriage bolts. You'll have to have some type of handle to open the hatch and, depending again on position, that could be as simple as a recessed, folding handle commonly used in travel trunks.
As I indicated earlier, the easier part is replacing the carpet with vinyl. The greater challenge could be making the hatch. Hope this helps.
