OK, I understand what you're trying to do now.. but I still don't understand why it's important for you to draw one large bitmap. You will lose a lot of flexibility but won't gain much from what you've told us so far.

You're basically writing a flip screen game I think.
In which a character moves from one side of a fixed 2D screen to another, then the screen flips as he enters another room.. like in Manic Miner and Another World.

Using your normal tile system is fine for this. You're not going to gain anything by cutting out the loop which renders the small tiles. It looks like it's doing more work, but it's actually quite fast. it also means that you can have objects in front of the player and behind if you also include the player in your rendering list.

Flip screen games are great fun and it's a shame there aren't many around these days.

One reason to have a single image as the background is if you want to employ a dirty rectangle system.. only updating the parts of the screen which have changed since the previous frame. This was done to squeeze every last drop of speed from very slow processors..

The other main reason is to save time if rendering your tiles takes much too long.. like if you've got some expensive lighting calculations which end up being a static image or if you're doing ray-tracing.. the obvious alternative to that would be pre-rendered backgrounds like in Abe's Odyssey.

So, unless you're doing one of those things.. (or something I've not mentioned which could be equally valid ) then I'd strongly suggest that you render the tiles as you need them. It also means you can have very dynamic parts to your scene.. lights flickering, candles, bats outside.. etc..

What you're trying is possible and the right thing to do... in some situations.