The Nazi's did destroy a lot of the evidence. The were very precise record keepers. We know from intel intercepts the number of dead Jews on a certain day, mid war. If the Germans had nothing, we would probably have very near an exact count of the killed, and their names. Have you read, IBM and the Holocaust?
http://en.wikipedia.org/wiki/IBM_and_the_Holocaust
It goes into a lot of the details of the records need to execute the Holocaust. Hitler was killing Jews who did not even know that they were Jews due to the 1/4 rule. A sixty year old man, who granddad has been Jewish as a child 40 years ago could still get you killed. You grandparents converting to another religion did not save you. They had to compile old census records from up to 100 or so years prior with list of local residents. When the SS came to town, they had a list of names and address of people to catch. This type of organization generates a lot of paperwork that is hard to destroy in its entirety. IMO, the Germans did an excellent quality job destroying records consider the rapidity of the collapse at the end.
As to bodies, the were digging up corpse to run through the crematorium but never finished the job. Himmler gave this job to a SS officer he did not like so much. In 1941, around 1 million or more Jews/Slavs/others were killed in mass graves. Some of the bigger sites were up to 33,000 bodies, but most were much smaller, as small as a few score. From memory, it was about 100 or so per site on average. It is just a lot of work to did up 10,000 sites in war zone, where the front is collapsing, when you are low on the totem pole for resources. For these sites, planting trees will not help, because the locals often attended and knew where the sites were located at.
To realistically shutdown the camps, Hitler has to stop using them months (I would guess at least 3) earlier than he did, and issue orders to kill everyone in an orderly fashion, then burn them and destroy the camp. If he did this, many 100K's more Jews/Slavs/others would not have died. IMO, this would have been unacceptable to Hitler and the SS extermination command.
Now, all this being said, If Hitler had started destroying evidence earlier, there would be a lot more Holocaust denier in the world, but no where near a majority. There would still be the trials, but they would not be as important.
And if there are no trials, it is because the trials were skipped and the high ranking Germans were just shot.