States do not own the Dwight Eisenhower Interstate System which travels through 48 states.
Your premise is wrong. The Interstate Highway System (which is in all 50 States and the District of Columbia BTW) is built by the state using 90 percent funds from the Federal Government is the road is free or 50 percent funds for a toll road. In the District, it is 100 percent Federal. The Interstates are built to a Federal standard but they belong to the state.
The Interstate Highways and many US Highways fall into a category called the National Road Network which was established in 1995 along with the establishment of the CDL (which is a Federal License for all intents and purposes) to make the movement of commercial vehicles across the country uniform. Before that, there were problems like Maryland with a 55' limit on I-95, the major East Coast Artery.
The recommendation for non-commercial RVs on the National Network is 65' rig length. The individual states generally accept the Federal recommendation on the National Network but can have exceptions.
Off the National Network, it is strictly what is in the state codes. A good example is in the Camping World guide in the post above. It states for Maryland the limit is 55' which is for Maryland roads. On the National Network roads, 65' is accepted and that is in the Maryland Codes 24-104.1 (m)(2)(i).
In Michigan, a 75' double RV trailer is allowed since 2007 so the Camping World Guide is a little behind.
I've already done the research!
means you have more to do.