1.  Install a heavy-duty electronic flasher.  Many truck chassis come new with a thermal flasher of the sort you see on passenger cars.  If the turn signal lights flash too rapidly and weakly, remove the thermal flasher and install a heavy-duty electronic flasher, available at auto parts stores.  Thermal flashers can be identified by their metallic body, which can't be seen through.  Electronic flashers have a plastic body, which is sometimes transparent enough for you to see the electronic devices inside.  Electronic flashers are better able to handle the extra load of the multiple turn signal lamps on tow trucks. 

2.  Use incandescent lamps instead of LEDs.  LED lamps, which have become popular for their distinctive look, use much less current to operate than regular incandescent lamps.  While reduced current draw is a good thing in most situations, it interferes with the operation of the flasher when you use LED lights as brake and turn signal lights.  Replace LEDs with incandescents and see if turn signal performance improves.  On tow truck beds with four red LED sealed lamps at the rear of the bed, for example, replacing even just one LED lamp on each side with an incandescent lamp will often add enough load for the flasher to work properly.

3.  Wire into main chassis harness, not into trailer harness.  One way the truck equipment industry has changed since I started in it back in the 1990s is that new chassis manufacturers are becoming more upfitter-friendly.  Chassis now come with trailer wiring and switch panels made new at the factory by the chassis manufacturer, to which the upfitter can connect loads.  For instance, a 1997 Ford 1 1/2-ton chassis had only one left turn and brake light wire where you could connect your wrecker bed lights, whereas a 2007 Ford 1 1/2-ton chassis has a chassis left turn and brake wire and a trailer left turn and brake wire, both original equipment from the Ford factory, both lighting up when you move the turn signal lever.  This proliferation of complexity and upfitter options provides more opportunities for the build job to go wrong.  If you wire your wrecker bed into a factory harness turn and brake light wire and the turn signals blink too fast, see if there's another, unused wire in the factory wiring harness that also lights up when you move the turn signal lever.  Turn signal performance sometimes improves if you run your wrecker bed turn signal lights off the factory chassis harness, instead of off the factory trailer harness.