I'm pretty sure "legally" you can tow anything you want with any vehicle, as long as the hitch set-up, chains, lights, break-away cables, etc. are all legal.
Even a small Chevy S-10 can pull a 20,000 pound travel trailer. (yes it can).... the question is, .... "HOW FAR?" will it tow before the transmission breaks or the engine explodes.
Another factor to consider is the braking ability of the tow vehicle. Does the vehicle have enough "umph" to adequately stop the trailer in tow? One would think the brakes on a trailer would be adequate to stop the trailer on it own. The fact is, that's not necessarily true. When momentum gets the trailer going, it takes and equal counter force to stop it! What happens (IF) the trailer brakes goes out.
Not too long ago I forgot to plug in the pig tail on the trailer. We got withing 20 miles from home when someone flagged me down as I was making a turn into a grocery store parking lot. He said my trailer lights were out. I checked. I forgot to plug in the pig tail. I had no clue! The truck braked the trailer well enough, I didn't even realize I had no brakes in the trailer. I plugged in and THEN I really noticed the difference ... DUH! This was the only time I missed plugging in the camper. Hope I never do it again!
Point being, my truck was big enough and heavy enough, that even without trailer brakes, the truck was more than sufficient to stop both truck and trailer by itself. Any vehicle that cannot do this (in my opinion) is simply too small for the trailer being towed.
Edit: I just read the first responded post. Maybe I misunderstood. If you overload the bed of your truck more than the rating of your license plate, yes you are illegal.
If you overload a trailer more than it's rated amount, yes, you are illegal.
In Indiana, my 3500 dually can be licensed at 12,000 pounds, 10,000 pounds, or 7,000 pounds. As the weight limits of my truck indicate a payload a little more than 12,000, I decided to license it at the 12,000 tag. I pay more for sure. If I licensed it at the 7,000 license plate rating, the entire truck and with it's payload cannot exceed 7,000 pounds or I'm illegal.
By the way, the weight of a trailer is not included in those weights. I learned this too. Weight limit applies only if loading something like, dirt or gavel, that does not have a separate license plate or wheels touch the ground.
