Yes Harry. Exactly, almost. One xml would for RTMM- if built correctly, starting with the base components and base scenery, specify layers for their insertion layer into the scenery order (IE: specifing layer 3 or WORLD for world-pieces), layer 20 ish or GLOBAL for global texture placement but yes works. But the only drawback is that IF all is in ONE xml for all of the pieces, you can turn it off or on. but ALL have to be off or all have to be on- there is no option to turn off certain pieces and keep others one. So if you want that flexibility there is none so you pick what you might want OFF. (and build those as separate xml so you can turn them off or on).
Not everybody wants all the pieces while others would want the entire RTMM package,, every scenery, every enhancement built and all of it either ON when being use, or OFF when not in use.
Henry is absolutely correct - RTMM or 3rd part add-ons still MUST be placed above ORBX (FTX-Regions and Airports) as an insertion point and you can't do that with an add-on.. but you can easily make a scenery entry IE: Third Party Start\scenery and us that scenery entry as the top insertion point which ensures that RTMM add-on stays on top.
Now if you also did a tongass xml, you have to set it layer IE: 159-160 to put it inbetween OpenLC and Orbx reiongs & airports. That's is where is has to be placed. (This is in lieu of using tongass low as the bottom insertion point. But to set up the scenery blocks correctly - you still have to set a bottom insertion point for correct layering. So create another scenery entry called - openLC add-ons\scenery (or anything like that) so you can set the bottom insertion to it.
"Orbx openLC placed below that "openLC add-on" scenery entry which creates the correct blocks for layering.
DONE.
Everyone does all this stuff differently, honesty there is no exact right way to do anything. Some don't work well with xml, while others do and there is some add-ons you want ON all the time - so fix them, but some not and although xml can turn things on or off- they are static, unmovable while other methods offer more flexibly. Anyways, to each their own way.