You've answered your own question.
I understand it depends on the State you are traveling.
If the state you're in requires it then you are required to have them. There is no reciprocity on braking laws unlike drivers licenses and insurance. You must be legal in the state you're in at the time. In WA, OR and CA my rig wouldn't need it but I travel outside those areas and I need it there.
Besides the laws of physics trump state laws.
2009 45' Magna 630 w/Cummins ISX 650 HP/1950 Lbs Ft
Charter Good Sam Lifetime Member, FMCA, SKP
RV'ing since 1957, NRA Benefactor Life, towing '14 CR-V