actualy there is a law in every state and provence, let us not get confused between the law and weather it is enforced or not. generaly if your unit is in good condition and your attitude is good when you get pulled over you'll get a try to be a little lighter next time.. in BC I only know of two private vehicles with rv's that have been ticketed (could be more) and they were grosly overloaded to the point where the bumpers were almost hitting the ground (not realy but you know what I mean)
in the trafic acts there are multiple tickets they "can" give to overloaded rv'ers, the key word is can. also you can be pulled over by Comercial vehicl safter and scalled if they decide it warents it. my cousin did get a ticket for being over weight with his 5th wheel in Alberta. the rcmp held him untill comercial safty brought their moble scales down and he cought a ticket for being 300 lbs over his rear axel weight.
the things is they are enforced so rarely that we don't think there is rules, just try towing a boat behind your 5th wheel into BC, if you make it to kamloops I'll buy you a beer haha