I use the annex under my RTT and a Luggable Loo.  It is extremely nice not to have to run outside in the middle of the night- especially when it is raining/snowing or just downright cold.  It can stink a bit in the summer heat, but it isn't overwhelming or nauseating so long as you keep the lid on and ensure you have a cross breeze though whatever space you're putting it in.  Bring gallon storage bags to put the toilet bags in (the toilet bags are self-contained, but you'll want that additional odor barrier once you toss them in your trasharoo that's probably baking in the sun.