Since you are in Ontario - here is the MTO listing of what licence is allowed to drive what type vehicle.
http://www.mto.gov.on.ca/english/dan.../classes.shtml
The older M135 CDN weighed about 13000 pounds empty so a G was enough if you kept the load reasonable. As I read it, you need the Z endorsement to operate a vehicle with air over hydraulic brakes but the GZ combination is allowed. Z endorsement can be earned in a weekend at community colleges, fairly easy to do.
A new (and in my opinion silly) wrinkle seems to have been introduced in July last year. A pickup may now tow a 5th wheel RV of over 4600 kg provided that the combined gross weight is below 11000 kg (and a couple more limitations). OK, but now for the silly part, if the trailer is anything except an RV the trailer is limited to 4600 kg. Same truck, same hitch, same driver, possibly safer in crosswinds due to less side area than an RV but different allowed trailer weight. Clear discrimination against CMP haulers. I don't make, enforce or fully understand the rules but I try to live within them and hate to see goofy rules introduced to silence lobby groups without thought to the implications. In case you are wondering I don't see how a M35 can be fit to the pickup definition (3 axles).
When in doubt, go to the rule book and read it. That way you have the same source that the police might use to charge you.... and remember to review it every so often because the rules can change (in either direction) as noted in the paragraph above