It's been said many times on this forum, you can't trust what anyone at the dealership has to say without verifying it, because they have a conflict of interest: They are trying to sell you a trailer!
The reality is that you would be overweight, but the odds of you having trouble would be slim to none as long as you kept your truck and trailer somewhat maintained. You would've slowly realized over a year or two that you didn't like the performance and/or handling of the rig as it was, and would upgrade trucks.
Most everyone is uncomfortable with driving a rig long before the rig is unsafe. There are some people who are blissfully ignorant to issues going on around them, but the fact that you are this concerned indicates you would recognize an uncomfortable situation long before it became unsafe.
