This probably doesn't apply to vehicles not registered in Wisconsin. If you're legal in your state with your license plates, Wisconsin probably will not give you any static when traveling through their state. But it won't hurt to contact them and ask:
The RV trailer must weigh more than the boat trailer. The only hangup you might have is when on a CAT scale if the weight on the RV axles is less than the weight on the boat trailer axles. The back trailer must weigh less than the first trailer, and the quickest way to check is axle weights. But the LEOs probably won't even bother checking unless the boat trailer is obviously a lot heavier than the RV trailer - or if you are involved in an acident and someone gets hurt.
Most states have a similar rule, but they don't require a permit, and are more restrictive in that the RV trailer must be a fifth-wheel or gooseneck design, not a tag trailer. Apparently Wisconsin allows the RV trailer to be a tag trailer.
Who would I even call???
Here's the application form. It includes a link and E-Mail address for more info.
I don't see it with a quick search, but if you're in Wisconsin then I'd start at the same DOT/tax office where you get your license plates. If they're not the right place, they'll know who is.
Any good advise when getting everything setup for this style of pull?
Spend some time and money on a CAT scale to be sure you have the proper weight distribution.
Many boat trailers have too much weight behind the axles resulting in less than 10 percent of the gross trailer weight on the hitch. That's a candidate for severe sway that could ruin your day. So I would spend some time on a CAT scale and move the weight around so I had at least ten percent of the boat trailer's gross weight on the hitch.
And that much weight on the back of a tag RV trailer might make it too light on the hitch too. So move stuff in the RV trailer until you also have at least 10 percent of the gross weight of the RV trailer on the hitch after the boat is hooked on (before you connect the weight distribution (WD) bars). Then be certain you have a WD hitch for the RV trailer. I'd also prefer a WD hitch for the boat trailer, or as a minimum a sway controller, but most boat trailers have surge brakes that do not play well with most WD hitches and may not even allow a friction-type sway controller.
I often tow a 3,000-pound box trailer behind my 5er. When the 5er does a jog, the box trailer does a HUGE
jog. So sway is a constant fear. I hook it up with a sway controller to help reduce the chances of an uncontrollable sway condition.