We seem to go around and around on this periodically.
The bottom line is that there is no single, unified standard when it comes to privately owned vehicles. Everything goes on a state-by-state basis, and to really be certain in any given state, you really need to consult the codes of that state. That being said, several places try to consolidate the information in tabular form; one of the best I have found is here:http://www.towingworld.com/articles/TowingLaws.htm
Now before anyone rolls out to correct me about federal highways, yada yada, let me point out that vehicles used in "interstate commerce" are subject to a federal override of state regulations. This is a matter dictated by the constitution, and you can read it there in black and white. Courts have generally held that the provision for interstate commerce apply ONLY to vehicles that (1) are declared commercial vehicles with commercial plates and (2) have been registered as carriers of interstate commerce, which nowadays is handled by the US DOT (the "ICC" that so many fondly recite was dismantled eons ago).
So, even if you stick to the Interstate highways and other roads considered part of the "STAA" network (a reference to the Surface Transportation Act, which, again, applies only to registered interstate commercial carriers), if you are a privately registered vehicle with non-commercial plates, you do not get to enjoy any of the commercial interstate exemptions to state laws.
For example, many states limit private trailers or semitrailers to lengths of 40', 45', or sometimes 48', whereas the STAA guarantees certain commercial vehicles access through those states to trailers up to 53'. Unless you are a registered interstate carrier, you must comply with the lower limit imposed by the state.
Contrary to popular misconception, there is also no magic "reciprocity" between states concerning vehicle length, weight, or other provisions. There is reciprocity for driver licensing, so, for example, I need no special license to drive my 48,000-lb coach in my "home" state of Washington, so I can also drive it in Texas, where, had I been a resident, I would need a Class-B over 26,000 lbs. Likewise, my Washinton license allows me to drive a 45' coach (even though I don't have one), so I could drive a 45' coach in California, too, even though residents of that state require a Class-B above 40' in length. None of this applies, though, to overall vehicle legality: in Washington, I can drive a vehicle combination up to 75' oin length, but such a (non-commercial) vehicle is simply not legal in California, no matter who is driving.
If you truly have a need for a rig that is legal in all 49 continental states, you need to take the "lowest common denominator" of the individual state laws. When we were shopping for our bus, we worked that out. Your coach can be at most 40' long, 96" wide, 13' tall, and weigh no more than 18,000 lbs per axle. If you had such a maximum-size coach, you could add a trailer no larger than 13' long (total including hitch), 96" wide, and 13' high.
"Mandatory Safety Equipment" is exempted from the calculations. So if you have, say, a 96" wide bus from sheet-metal to sheet-metal, the marker lights, reflectors, side mirrors, etc. can protrude beyond that (but not
such items as awnings, patio lights, step rails, etc.). Normal safety bumpers can extend beyond the 40' length limit, however specially modified bumpers to carry additional load or equipment such as winches, etc. can not.
Note that, in practice, if your vehicle looks "kinda commercial," you will likely not be stopped in, say, New Jersey for having a 102" wide bus, or a 20' trailer behind it. But you could, and you could not only be cited, but also sidelined and forced to be towed, or have whatever is excess (such as the trailer) moved separately.
I suggest you comb through the table linked above, and figure out what states you need to drop from travel plans to keep your combination rig compliant. New Jersey and the District of Columbia are two that are very restrictive, but easy to just go around.