Can rough roads create a suction from the water tank vent?

In a properly designed and built system the answer is NO.

However... if you fill up the water temp in the tank is generally approaching sixty degrees (normal tap water) and you may be... Well the outside temp is 81 here when I parked (Cooling off as it gets dark) Water expands when heated EXCEPT at the freezing point. (absolutely unique in that "Except" no other material exhibits that expanding when freezing).

Thus the tank, if filled, WILL overflow as it warms.