crushing a car is a better option than hiding it under some twigs and boards.
No. It fucking wasn't, dude.
Salvage yards don't have crushed cars sitting around them. Crushed cars are useless for salvage purposes (everything is crushed and parts can't be picked from them), so they are sent to steel recycling for money. But if he had crushed the Rav4, he couldn't have sent it to be scrapped and recycled because you can still easily sometimes tell the make and model of a crushed car. Someone would've noticed, "Hey, Steve Avery sent us a crushed green SUV right around the time Teresa Halbach turned up missing."
Which means he would've had to leave the crushed car sitting on the property. Which they do not do at a salvage yard. A crushed car sitting around on the property would've been even more conspicuous to his family than just another SUV back on the property somewhere. And it wouldn't have been much easier to hide. Like I said before, crushed cars aren't reduced down to some Bugs Bunny style cartoon cube. They are still pretty big. Google some pictures for crying out loud.
Crushing the car was not a viable option for getting rid of the vehicle. Period.
There were only two ways he could've gotten rid of that car:
Cut it up with a cutting torch by hand and scatter the parts. This is fairly inconspicuous, but would've taken an extremely long time during which someone could see him.
Dump the car somewhere. My guess is he intended to do this, but stashed the SUV in the rear of the property until he could work out a way (since he would've needed another mode of transportation to get back from wherever he dumped it).
Either that or the dumb motherfucker just didn't think far enough ahead to figure out what to do with the vehicle at all. He did have an IQ of 70 after all.