Boy, it gets interesting quickly if you try to find a definitive source of info about towing laws in various states.
Brake-buddy has an intuitive way of presenting the basic weight info at
http://www.brakebuddy.com/towing-laws . Good way to see how much the laws vary. But are we talking about towed vehicles or not? They seem to suggest they are with the line 'If what you are towing exceeds the weight, an auxiliary braking system is required".
Towing World has a less-flashy map (at
http://towingworld.com/towinglaws.cfm)
but much more detail in the individual listings, including a line "Brake Laws Towed Cars"-- and most of the states I checked said 'No laws regarding'. But can we trust it? At first glance, this appears to be a good place to find info. But if you then start reading the reference notes at the bottom of the page, it doesn't make a lot of sense. This looks like a bad import job from a much more extensive database which could be a lot more useful to us.
As an aside, I wondered if there's an 'app for that' for my ipad. That led me to 'State Lines' in the iTunes store. It was a $5 purchase but, hey, I'll skip my coffee down at FourBucks today so it was only a buck. This app has pages you scroll between in categories "Shopping" (various tax rates and time zone), "Protection" (seatbelt, child safety, motorcycle, and bike-helmet laws), "Bans" (cellphone, texting, smoking, radar detectors, and leash laws), "Speed Limits" (defaults), "Driving" (left-on-red, right-on-red, u-turns, keep-right and misc), "RVing" (Rest Area Overnight Rules, State Park fees on Top of Camping rules, max lengths, Safety Devices required (chains, breakaway, fire extinguisher, flares, etc), Trailer Brakes (just a weight number), Max Tow Speed, Triple-Tow Allowed, What You Can Ride in While Towing (such as 5th wheel, pickup camper, travel trailer, etc), and "Alcohol" (where various classifications sold, hours, open container laws, etc.) I note the developer recently said he's doing a complete rewrite and the info is getting a little old. He normally reviews and updates every six months. Bottom line: no real help specific to towds but useful for wandering multiple states.
-OC