The way a TT toilet works you're just opening a direct vent into the holding tank. With normal use, a holding tank won't hold stuff long enough to break down and turn into gases. What little does break down is vented.
If you were to plumb a TT tank to a septic, you'd be letting all the gases in that septic back into the trailer every time you flushed. And unlike a holding tank, you can't dump a septic to get rid of the stink.
If you can't find a residential toilet that fits, I'd recommend keeping the holding tank, and just plumb from the dump valve to the septic. If the tank get's full or things start to stink, just a matter of pulling a handle.
One option to look into for space saving is a tankless toilet design. Having the tank on the back of the toilet is what generally eats up space, but I've seen some pretty slim designs without tanks in public restrooms. No idea where to buy one though....
There's no use crying over spilled milk, unless it's on your keyboard.