I think the part you're missing is the salesman was asked and answered the question. Was it the responsibility of the salesman to volunteer the info? No! Must the clerk counsel the person buying alcohol? No! But if asked what happens if I consume this alcohol and then drive and the clerk tells me I will be fine then the clerk has to live with themselves when I get a DUI or kill myself or somebody else. Will they be legally responsible? Probably not.
Don't get me wrong, the buyer was a fool for not knowing what their TV is rated for, but the problem I'm having is the salesman LOOKED at the TV and told these people they are fine, knowing that the sale of that particular TT depended on his answer. I'm sure there was a TT on the lot that they could safely pull and he could have still made a sale.
What is the difference if he told them the trailer had a 100 gallon FW tank but it really had 30 gallons? After all, he just needs to make a sale and can say anything that gets it sold. He took advantage of this young couples ignorance and that is just plain wrong! At the very least he should have said he doesn't know and it's their responsibility to find out.