I had the same problems. Low RH and struggling to keep temps under control. I tried a lot of things but what I finally did was turn the thermoswitch of the airpump a bit higher, which makes for higher temps in my tent, but raised the humidity since it is off more and doesn't keep Sickinghe thé RH out. I did established the temp that my plants start showing heatstress and made sure it stays just under it at the max temp. Also I have Windows open so the tent get's cooler air and the room temp where my tent is, is lower so it helps cooling inside too. And I have a tray filled with water in front of a vent so there's more watervapour being produced.
If you have too much RH you can do the opposite I reckon. Lower the temp the pump works and it will evacuate more watervapour and then try to keep your temp at the right spot by tweeking the air intake and climate of the room you have the tent in