Those are the outside dimensions, I think the cut-out is about 19x9" or somewhere close to that. That is the 'new' style access hatch with attached 'piano' type hinge. I have the older RM183 hatch with the detachable 'hurricane' style hinge on my '84 Scamp
13. The door was missing when I bought my Scamp
but the frame was still in place so I went to considerable effort to find the same RM-183 in order to not hafta replace the frame with the new S-183. Either will work fine & the S-183 may actually be a little better as you're not likely to lose it if the latch disengages while going down the road. The used RM183 that I obtained has been 'well used' & is not as sturdy as the perfect condition one that I have on my old P/U camper so I'll probably use that one for travel.
A few pix of the hatch opening, details of the 'hurricane' type hinge (both on door & frame), & the used RM183 door