GML [Solved] Reading room data, but not the current room

[Electros]

Hi - the basic premise is that I am trying to;

1. Create rooms as individual chunks.
2. Iterate through the desired rooms, and save the data (instances, x, y for now) in each to a data structure (e.g. ds_list, ds_grid)
3. Create these chunks in the current room when called upon, using the saved instance and positioning data

I'm falling down currently on section 2 - I thought I would be able to use room_add() to create a room, then room_assign() to load room data in, read out and save whatever data I need, and work through the chunks. The below code is giving me 0s for everything.

//Create the logging variables
var pRoomWidth = 0;
var pRoomHeight = 0;
var pRoomInstanceCount = 0;

//Create the parser room
parserRoom = room_add();

//Assign the chunk data to the room
room_assign(rm_Game_Chunk_1, parserRoom);
with(parserRoom)
    {
    pRoomWidth = room_width;
    pRoomHeight = room_height;
    pRoomInstanceCount = instance_count;
    }
show_message("Dimensions: " + string(pRoomWidth) + ", " + string(pRoomHeight));
show_message("Instance Count: " + string(pRoomInstanceCount));
   
//Assign the chunk data to the room
room_assign(rm_Game_Chunk_2, parserRoom);
with(parserRoom)
    {
    pRoomWidth = room_width;
    pRoomHeight = room_height;
    pRoomInstanceCount = instance_count;
    }
show_message("Dimensions: " + string(pRoomWidth) + ", " + string(pRoomHeight));
show_message("Instance Count: " + string(pRoomInstanceCount));

Should this method even work, i.e. am I meant to be able to access and pull data out of a room created and then assigned which isn't the current room, in which case how would I go about that?

Or is there a better way I could / should be going about reading room data not from the current room? My other thought was to setup a persistent logging object which then went through each room chunk and logged the data as it went. Could be done once on game load.

[Edit: Solved - went for the logging object method that flicks through all the target rooms recording the required data on game load]