Check this site out: http://ops.fhwa.dot.gov/freight/publ.../index.htm#cmv
According to FEDERAL regulations: "Recreational vehicles are not CMVs subject to Federal size regulations" - a CMV is a Commercial Motor Vehicle. ost truck motorhomes are However, each state has it's own laws. If you are on the National Network - the interstate highway system - there is no length restriction and furthermore, all states must allow reasonable access (up to 1 mile).
The states are totally different. Here's a pretty good site with a comprehensive list of state laws pertaining to RV lengths: http://www.wecamp2.com/size.html
. Please check with your state as this website may not be up to date.
As far as the national network (and reasonable access of up to 1 mile) The overall length is measured end to end with this exclusion: Devices excluded from the measurement of the length include 1) those needed for loading or unloading that do not extend more than 24 inches beyond the rear of the vehicle; 2) resilient bumpers that do not extend more than 6 inches beyond the front or rear of the vehicles; 3) aerodynamic devices that do not extend more than 5 feet beyond the rear of the vehicle, provided they do not have the strength, rigidity, or mass to damage a vehicle or injure a passenger in a vehicle that strikes a trailer so equipped from the rear, and do not obscure tail lamps, turn signals, marker lamps, identification lamps, or any other required safety devices, such as hazardous materials placards or conspicuity markings; and 4) non-property carrying devices that do not extend more than 3 inches beyond the rear of the vehicle. All devices at the front of a semitrailer or trailer are excluded from the measurement of length. Tarping systems for open top trailers or semitrailers are excluded provided no part of the system extends more than 3 inches from the sides or back of the vehicle. For more details on these exclusions, see 23 CFR 658.16 and appendix D to 23 CFR 658.
Again, this pertains to the national network. Each state's laws may differ, so you'll have to check with the individual state.
Hope this helps a little.