Without doubt its due to low insulation resistance (I/R) on the heating element/s, its a common fault with all tubed heating elements when they get old/tired. If you have the means to test the I/R ie. with a Megga anything with an I/R to earth below 10Mohm is on its way out. Its breaking down and allowing current to flow to earth which in turn is tripping the RCD. They trip at about 30mA (0.03A) so you are loosing at least this much current to earth. Unlikely to be a short anywhere or the fuse/MCB would be more likely to blow/trip. Take the old one out, shouldnt be difficult and take it a spares agent along with an exact copy of the details on the makers nameplate inc type and serial number so they can supply an exact replacement ie; style, power in w or Kw, and voltage. Some are 240v some are 110v, depends if there are 2 or more and if they are configured in series or parrallel. If you have 2 or more elements in the machine it makes sense to replace them all at the same time. a new one in circuit will increase stress on an old one and dramatically shorten its working life.