Some must have very flimsy bodies . Cars, in particular, use windscreens as part of the structure so you would expect similar of an A class motorhome. A class motorhome builders give little thought to the consequences of other parts of their design so why should screens be any different. If there is a reason for still using rubber it will be based on economic requirements nothing else. Screens fitted using adhesive are unaffected by slight variations in the size of the body aperture caused by the moulding process and are much less likely to leak. They are somewhat more difficult to remove but very much easier to install.I believe there is more stress on a glued screen if there is any movement in the body, most of the big A class screens are on rubbers, must be a reason.
Martin