Look up the hotel on the internet. See if their breakfast and dinner menus are on their website. If they are, you can make your choices in advance.
Most hotels offer lighter options for breakfast. The one I regularly stay in has a lovely fruit salad, or sometimes I have poached eggs. If nothing on the menu looks right to you, explain that you are on a low-fat diet and tell them what you would like.
Dinner - the same. Ask for what you want. The steak without the sauce, the salad without the dressing, the fruit salad without the cream. Ask nicely, you are the customer, and there should be no problem.