Here's what the sample seen in the first screenshot does on room start in a controller object:
var json = scr_rts(); // generated from rooms starting with rt_ room_pack_blank_object = obj_blank; randomize(); with (obj_room) { // obj_room is placeholder object // pick a random room name from the map: var name = ds_map_find_first(json); repeat (irandom_range(0, ds_map_size(json) - 1)) { name = ds_map_find_next(json, name); } // and load that: room_pack_load_map(json[?name], x, y, room_pack_flag_instances|room_pack_flag_tiles|room_pack_flag_sprites); } ds_map_destroy(json);
Also make sure not to accidentally insert your outer room into itself (you can specify the filter-prefix for this reason)