I'm going to dissect your post a little bit and throw in my two cents...
Since it appears this thread is still active, I'll ask my question here.
I've pretty much decided that my first RTT will be Howling Moon. I plan to mount it using a
http://www.tracrac.com/ on a Ram 3500.
That will work just fine. We are a TracRac Dealer and we can help with this as well.
I want to mount the tent so that it opens toward the rear of the truck.
You'll need to watch the size of the tent (read width). Since the width is going to determine how it is mounted, typically up to 1.4m tent is mounted so that it can be mounted with the opening in the rear of the vehicle. Think of a sheet a plywood, once you get too wide, it visually starts to look like a spoiler/wing. Once you get above the 1.4m size it becomes more rectangle shaped with the longest side being the side that opens.
I'm assuming from pics I've seen that the HM tent can be mounted to opened from either the side or to the rear of a vehicle.
That is correct, the RTTs come with mounting rails, it is basically a T-slot channel that captures the mounting hardware. This allows the tent to be mounted on several different configurations.
So, my question is...are there pre-drilled holes on the bottom of HM tent bases so that the mounting rails can be attached for either direction, or are people just modifying the bases to work for their desired set-up? ...
In your case with the TracRac and wanting to open off the rear of the RAM you would not need to do anything differently other than your front load bar would not be as far forward as the cab.
If you have any other questions let us know.