From the Sask. website regarding farm plates;
"Vehicles registered with farm plates can be driven for personal purposes by anyone the farmer chooses to allow, without time or distance restrictions. However, heavy vehicles with farm plates operated outside of Saskatchewan must comply with the requirements of the jurisdiction they are in."
We've had questions raised before by LEO's and assorted DOT's when hauling horses across the States. In almost every case they wanted proof of some kind that they were ours and not being hauled for others for money.
The other 'yardstick' they use to judge commercial use or not is the fifth wheel itself, if its a standard commercial unit they look at you a little sideways, but if its a smaller 'RV' or even Holland mini-fifth they realize that you can't haul a commercial OTR trailer and pretty much leave you alone.