As mentioned above...
As long as its primary use is not a piece of "mobile plant" (crane, cherrypicker, generator), it can tow.
How much it can tow is dependant on manufacturer's info, Gross Train Weight (if it was ever assigned one in a past civvie life?) and sensibility/looking "OK"!
Being a wrecker, there should be no probs weight-wise (considering the weight of a caravan), just make sure the hitch is well made and up to the job (it pre-dates construction and use regs), and that the lighting is up to scratch (bit trickier as all the caravan behind the WLF would need for being towed is triangles and tail-lights, but would be sensible to have indicators and brake lamps plumbed in as well). It goes by the age of the towing vehicle and the regulations governing it, but if some monkey following you decides you should have indicated, grey area.
Find/build yourself a mammoth 1940s caravan with matching colour scheme to tow behind! :cool2:
Best of luck and show us photos!
Alec.