If they planned this, I doubt there was a quick dump anywhere. More likely, there was a disposal in a pre-meditated location that makes it difficult, if not impossible, for the body to be found or recovered.
My impression is that for some time now LE has believed they would not recover Heather. So, just as SM's pay phone location and call time were surely not a coincidence, successful disposal of Heather's body was not just luck. And, I can't see TM concocting a murder, pulling it off, then not having a pretty specific manner of further demeaning and punishing Heather with punctuation that would be gratifying to TM.
So whether they had help, or managed to dispose of her body on their own, I think Heather is well-hidden where TM perceives control over the chances of her being found. As proud as I believe she is of her handiwork, the last thing she wants is for the body to emerge.
I personally think Heather is contained in something. Whether or not the site is farther away than the immediate area may depend on whether or not someone helped. And I think if LE knew or had ideas about the M's movements right after the murder, we would not see them searching water, then searching local areas, talking with LE in another county, then calling Florida...
So I don't think authorities really know what the M's did after Heather was killed. It's a case of my guess/your guess is as good as their guess, or anyone else's.
So, where might she be? What would make sense to TM?
Are there wells in that area? Are there other structures on farms or other property that would make it nearly impossible for anyone to find Heather without benefit of a tip? I know we've discussed septic systems. How accessible would those be, given locks or other issues?
I've just never felt she was in the water or on the top side of the ground because those types of disposal are too risky.