In the OP the size of the trailer was unknown. And guessed to be in the 28 to 30 foot range. If the manufacturer declared and advertised it to be 1/2 ton towable, that is a design feature of that trailer. And the sales persons responsibility (job description) is to advise a customer of the products features and benefits. It is NOT the salesman's responsibility to doubt the documented product specifications. Especially to the customer. If he does that he is not doing the job.
Does not qualify him as a liar, or deceiving the client.