GST tax rate on actual hotel room rent or declared room rent
Tax rate on hotel rooms is based on the declared rent. For example, if you have rent price list and rent as per price list is 8000 then you have to charge tax 28% regardless of amount you charge to guest.
In your case if declared rent per day is Rs. 8000 but you charge only Rs. 5000 then you have to charge 28% on 5,000.
Rent for hotel room is 5000 but collected more because of extra service or extra bed
As already said tax rate will be based on pricing list. So even if you collect more than price list you have to fix GST tax rate based on displayed pricing list. You need to charge GST at 18% on room rent collected since your quoted price was 5000.