Well, profit is not always related to user experience. There are a lot of factors that contribute to it, so you might want to take those into account as well.
Creating a good experience for all the customer touchpoints can definitely increase the brand value and increase the sales in the long term.
You can definitely start from current guests, but don't stop there. You should have a strategy and target a specific group of customers, then start to interview potential customers that fit the description.
Anyway, this is a large topic and it cannot be covered here. What you can do with current guests is something similar to an exit interview, something you would do around checkout. Ask kindly for 15 minutes of their time before checkout and invite them in the administrator's office for a chat. Make them feel important through the whole process and they will probably open up and give you honest answers.
As far as selecting them for the interview, any strategy is ok. You can do random selection, or split them between demographic categories e.g. men, women, young, old, rich, poor, high education, low education, etc.