Another way is to move your plans onto one layer. Go to layers then classes and make all visable. Then > Organize>Layer options>Active only. Then go thrue your layers cutting and pasting on one layer. Each layer you cut and move into the one selected layer, just move it over right after you paste, say about 150 feet. Put you floor plans in the middle, elevations above... Purge unused obects and layers. ...Then export the one layer with everything on it into one dwg file.