Do I need an attenuation cable between my audio recorder and the RCA out of a DJ's mixing board?
Copied from the Tascam manual:

Analog audio input/output ratings
MIC/EXT IN jacks (supporting plug-in power)
Connector: 1/8” (3.5 mm) stereo mini
Input impedance: 25 kΩ
Nominal input level: –20 dBV
Maximum input level: –4 dBV

This Tascam doesn't even take nominal consumer level without distorting (-10dBV). 
Most likely on an RCA output from a mixer you'll have -10dBV, so you'll need to pad the level by at least 10 dB (nearly 22dB if the DJ mixer outputs professional +4dBu).

But, the Tascam lets you trim level before recording, so it looks like all the pad you need is in the Tascam itself.
Just make sure you set that beforehand to a level around the -12dB mark on the Tascam's meters and that you take out the Auto Gain, which always ruins levels. Check the manual which explains how to do these clearly.