Both Badaling and Longqing Gorge are located at Yanqing city, so I think it will be more convenient to visit Longqingxia from Badaling than Ming Tombs. You may take bus no. 919 from Badaling back to Nancaiyuan bus stop of Yanqing city, transfer to bus no. 920 to Longqingxia. Then back to Beijing city, you can take bus no. 920 back to Yanqing first, and then transfer to bus no. 919 to Deshengmen. Remember to confirm with the bus driver ot the conductor whether you take the right 919 or 920 bus, since there may be more than one 919 or 920 bus.