This is not the salesman fault
Maybe not entirely, but I've experienced and read too many instances of salemen saying "Oh your truck will tow that no problem!" ONCE has a salesman said to me "too much trailer for your truck"
If an RV shopper is a newbie and doesn't know the 1st thing about towing, a responsible person who happens to be an RV salesman SHOULD have the ethical where-with-all to say "lets see what you're planning on towing with and run some numbers (specs).
I went with a buddy of mine a couple of years ago who was a newbie to go look at trailers. He has a Expy EL. We were looking at a 32' Jayco Jayflight with a huge slide that was going to be just too much for his rig. But here comes the salesman. Knowing the tongue weight alone would be prob 850 DRY I had him ask the salesman about the tounge weight. "Oh this wouldnt be more than 600lbs, your Expy will handle this unit easily." I stepped in and challenged his assessment. He ran into the office then came back and sheepishly said I was right.
Yes, the buyer has to do his homework, but what some salesmen will say and do just to move a unit is flat out ridiculous and wrong. Those guys need to do THEIR homework also.