Hi,
Letters like this need to be short and to the point. The fact that you enjoyed good service there is not important. If you mention is, it should be after you say what you need.
I have just returned from a weekend stay at your hotel and realized that I left my bag behind. I stayed in room RM110 in Phase II (I have no idea what that means. Here, we just say "Room 1305.") It is a small black leather bag, just a bit smaller than a notebook computer.
Please let me know if you find it, and how I can arrange to have it sent to me. I would appreciate it if you would contact me at [provide telephone number] either way.
I had a wonderful expererience at your hotel and I hope that my bag is found, so that I can remember the trip as being a complete pleasure and not marred by my forgetfulness.
***
The manager doesn't care that it was a holiday. He doesn't care that it's your cell phone or your landline. Just provide the basic facts.