Sounds like you're asking if the roof rafters will hold the weight of the cab. That's really a question for a structural engineer but I'll give you what I think (just a mechanical engineer). If the roof was originally designed with trusses, the bottom chord of the trusses was in tension and kept the weight of the roof from letting the rafters push the walls out. Unless the 2x10 modification replaced the ability of the bottom of the trusses to keep the rafters from hinging at the peak, I'd be real careful about it. If the 2x10 sandwiching the rafters make up a triangle at the peak with a significant horizontal base dimension, then maybe the modification was correctly engineered. Although if you've had snow build up on it and it survived that, the weight of the cab would probably be equivalent and you wouldn't bring the roof down, at least till winter with the snow load if you left it hanging that long.
