Retrofitting a roof ladder to a motorhome ?

Now looking at a Hymer 644 1993 campervan. Has no roof ladder which would be a necessity for us. One source told us that it is not good to retrofit a roof ladder, better to go with a model that already has one as they would have been manufactured with reinforcement in the area to carry same. Is this true ?

I think it probably is true. I don't think it would be that easy to fit something that would sturdy enough and look reasonable. I have a ladder on mine and only go up there for maintenance and even then I am careful to avoid damaging the structure. I don't think most of them are designed with the roof to be used as a platform in mind.