You've been so strong for your family, while several things are going on. Let's address each one.
First off, your dad's situation: From what you've told me, it
certainly sounds like he is approaching end of life: sleeping more than awake, limited appetite, end-stage renal disease. Your main role now is to keep him comfortable. Do not force him to eat or drink more than he wants, even if it seems like nothing. You may see further changes in breathing, skin and nail discoloration, and mental disorientation while the disease continues to progress. His body is slowing down.
If he says he is in pain, ask him to point to where it is, describe if it's a throb or a piercing pain, and when it happens. With this information, you should be able to assess if you can help manage it with Motrin or ibuprofen, or if you'll need to ask his doctor for something of prescription strength.
As for the lung drainage, you may need to have a frank (and difficult) discussion with your mom about that. Which of you have Healthcare Power of Attorney to help make his care decisions? Have you two talked about his Advance Directives? That is, do you know what your dad's wishes would be at end of life? For example, if he has a Do Not Resuscitate (DNR) order in place, you may want to rethink drainage and begin palliative care (this is the end of curative care and continuity of comfort care) to instead ease him to a peaceful passing. This is not taking his life, only easing his suffering. I know this sounds harsh, but remember, it is the cancer that is taking his life.
If he does not have a DNR order in place, you may want to consider doing so. Here's a website to help: www.nlm.nih.gov/medlineplus/advancedirectives.html. If he is still lucid, you will need to discuss this as well as other end-of-life decisions. This is so hard -- I know from having been through it with my father -- but you need to understand what he wants and act as best you are able.
To help your mom, gently include her but know that you cannot force her out of the denial stage. It has to come from within her. No doubt she has hope -- she has seen him surpass a prognosis before, so why couldn't he do it again? You know the situation is different now, but it may seem the same to her.
Both you and your mom are prime candidates to experience anticipatory grieving, when you are grieving for what has already happened and that you fear will happen in the immediate future.
Be aware that you are also a prime candidate for Compassion Fatigue, where caring stresses begin to have a measurable physical and mental toll beyond the emotional toll. Some caregivers have even ended up in the hospital, due to malnutrition, lack of sleep, dehydration, and broken bones -- all related to caregiving responsibilities. We obviously want to avoid this.
Here are some ideas: Delegate as much as possible to give yourself a little breather. Involve people to help and listen. Do you have a chaplain or social worker or best friend that will let you vent? Ask your oncologist's nurse, nurse practitioner or office manager for suggestions on what public services are available to help as well (like caregiver support programs, free shuttle services, prescription delivery, etc.).
Make sure that getting a good night's sleep is a priority. You may have to give some things up for that, but the rest of you can function better when you're a little recharged.
As another resource, my website (www.CompassionateCaregiverOnline.com) has handbooks on "The Journey of Grief," "Understanding Hospice and Palliative Care," and "A Cancer Patient's End of Life" -- all three of which could help you.
Hang in there; his situation will not last forever.