Ahh, problem solved, i had to quote the variable so it's outside of the sed call.
#!/bin/bash
sed -i 's/name="ServerPort"\s*value=".*"/name="ServerPort" value="'${ThisService_GamePort}'"/g' serverconfig.xml;
sed -i 's/name="ServerMaxPlayerCount"\s*value=".*"/name="ServerMaxPlayerCount" value="'${ThisService_Slots}'"/g' serverconfig.xml;
sed -i 's/name="ControlPanelPort"\s*value=".*"/name="ControlPanelPort" value="'${ThisService_QueryPort}'"/g' serverconfig.xml;
sed -i 's/name="TelnetPort"\s*value=".*"/name="TelnetPort" value="'${ThisService_RConPort}'"/g' serverconfig.xml;