Where are you going?
New Jersey and Oregon are 50 feet max. Kentucky, Maryland and West Virginia are 55 feet. Alabama, Connecticut, Delaware, Florida, Illinois, Indiana, Iowa, Massachusetts, North Carolina, Rhode Island, and Virginia set the length limit to 60 feet. Most of the rest of the states are 65 feet, assumed to be a Federal standard for the Interstate and designated highway system, but you sometimes have to get off those roads. A few states go to 70 or 75 feet, which applies to the Federal system in that state.
In states with limits more restrictive than the 65 feet standard for designated Federal highways, you are often allowed some off-highway access, one to three miles. This is mostly to allow commercial truck access to terminals and service destinations, but RV destinations might be that close as well.
You can usually buy a permit to go longer (we must move around wind machine parts, oil field service rigs and ICBMs from time to time) but oversize loads are restricted to specific routes and often require escort. Some states go beyond 100 feet on designated highways with commercial permit yet unescorted (they boost combined weight limits as well) but these are options not useful for a RV you want to take anywhere.
I would target 65 feet overall and be prepared to drop my tow where the limits are more restrictive.