As usual the save/load routine has involved more work than I thought. I've reduced it to the hardest part which is saving the level map. The old map with individual 'byte' maps for each layer was easy to save, because the byte map had a built in RLE (run-length encoding) packing. The estimation for 50 levels each average size of 100x100 tiles is 3,5Mb of space unpacked. How much is it compared to modern save files I don't know (I don't play PC games (other than roguelikes)), but it would be much less when compressed.
As expected the compression routine must be custom created to be compatible for both the save file format and map format which contains four mandatory variables to be saved: the tile itself (terrain), fov value (known tiles), mask and room id. These four variables can't be reproduced from any other data. The mask value is mostly used in the level's creation process, but it might just be used during the game in pathfinding situations.
RLE in theory is easy to implement, but the custom data structure and save format makes it somewhat more complicated. RLE also works quite nicely with typical dungeon levels of rooms and other areas with mostly 'empty' space (walls) between them.