Should I be worried about an RV that is too long?

I wonder if there's a website somewhere that you can pick your truck and it'll give you the maximum tow ratings and so forth. Or enter what you know about the truck and it'll calculate things for you. I have a spreadsheet that I put together but there are a lot of variables, and until I actually own something and pack it I won't know exactly what I have, so to speak. And then it's too late, heh. I should just get a Max Tow 3500 Mega Cab and be done with it. :)