Your most difficult thing for you to overcome will be ( reason to Return ) Does she have a job ? how long have you known each other ? how do you keep in contact. How many times have you been over to see her
All these are questions that the ECO will be looking at. You say she is renting from that I take it she does not have any assets in Thailand..
You could say that in the future we are planing to marry and that I would ensure she keeps to the Visa restrictions and returns as you would not want to jeopardise any future application.
But I do think it is going to be difficult without a solid reason for her to return to Thailand.
This is my humble opinion. No doubt others will come back with advice.