My family had a car dealership years ago. I know my dad never lied about weights and such. I suspect at least one salesman did if it made him the sale.
With RV salesman, I have found that they will simply always try to get you in the biggest RV on the lot. I know someone who went in to look at class Bs and walked out believing she needed a 40ft MH. When she told me, I asked her how she was going to camp at her favorite campgrounds that could only fit under 28ft. I asked her why she needed all that space when they had just wanted a bed off the ground. Fortunately for her, she didn't have the money at the moment and hadn't signed anything.
Like most things in this world, I think you need to do your own homework, and if you find a salesman you trust (and has proven him/herself) that you stick close.