tom O, the rules for commercial vehicles also apply to recreation vehicles. These allow 53' trailers on all National Network roadways (interstates and federal highways), plus "reasonable access" to/from fueling stations and delivery locations. (Information can be found here: https://ops.fhwa.dot.gov/freight/publications/size_regs_final_rpt/index.htm#rv). Individual states may limit trailer and/or overall length on smaller highways and local roads.
Are the shorter-trailer rules ever enforced? Sometimes.
I know that California has a lot of state highways where trailer length is limited; also the distance from kingpin to the center of the trailer's axle group. And these get enforced! So be informed before pulling a long trailer there.
- Old KW Guy