Good morning! I'm Dr. Sara - I'm a licensed veterinarian who works exclusively with dogs and cats. I'm sorry to hear that Luke isn't feeling well lately.
I'm very concerned with your description of what's going on with him. The first thing that comes to my mind when we suspect fluid in the belly, pale gums, and weakness, is internal bleeding. The fluid in that belly could be blood - the most common cause for this is a tumor on the spleen that has ruptured and begun to bleed into the abdominal cavity. Note that as long as the blood is inside the abdominal cavity, it does not come out in the urine or feces, so there will be no external loss of blood. Splenic tumors are typically quite easily diagnosed with a physical exam, blood work, and an X-ray or an ultrasound. If the mass is big enough to palpate on physical exam, sometimes the blood work and imaging is just to confirm our suspicions. If there is a bleeding splenic tumor, the vet will discuss surgery (in the context of what else the physical exam shows and the condition of your pet) to remove the spleen and mass. Unfortunately, a good number of these tumors are malignant, so even removal will only buy you a few months. In other cases, the mass is benign and surgery is curative. The only way to know for sure is to remove it and have it biopsied.
There certainly could be other things going on too like diabetes, kidney disease, GI parasites, or other systemic diseases - with what you're describing I would call your vet and have him examined ASAP.
Please let me know what other questions I can answer for you :)