Just because someone is an "engineer" doesn't mean they know what they're doing in a circumstance like this.
He was way over his head on this one
A smart engineer would start by contacting the factory discussing the intended project with the engineers that designed and built the trailer. That probably would have ended this project before it started.
That kind of weight that far behind the axles has to have negative consequences. Toyhaulers are made nose heavy when not carrying toys to compensate for the weight of toys. TT's are not typically made to carry toys in the back.