So do you have an exhaust fan? (A fan to suck old stale hot air out?) Leaving the vents open to let in fresh air won't really happen unless there's air being sucked out. Also extra fan inside the tent will just blow around the old hot air.
Easiest fix, buy a decent, inexpensive exhaust fan and ducting, and suck that air out and get rid of it away from the tent, leaving vents open to get fresh air in. If that's not an option I have seen people rig up regular fans as exhaust fans but it's not ideal.