M.G.
R.,
I am sorry you are having such a hard time with your family. I don't envy your situation at all, your parents (particularly your dad) seem to be taking advantage of you and your kindness. I honestly think the only way you can solve your problem is to put your foot down and refuse to help your dad. As selfish as he sounds, I'm sure he will be offended, and throw a child sized fit, but he is the parent here, not you. You have a family that you have to think about, and focus on. Your husband and children are sure to notice the time and effort you are putting into your fathers situation, and it could cause animosity towards both you and your father.
I guess the only thing I could suggest to do to please your father is to tell him you and your husband will set aside one day of the week to help them. wheather it be a ride to the foodbank, or help around their RV. As far as Christmas goes, get him what you want to. not everyone, especially adults, get what they want every year, and if he isn't grateful for whatever you get him, then he really is an ungrateful child, and you should really just cut him off. It sounds like you would be better off without the stress of raising another child.
Again, I'm sorry you have to go through this, and I hope it all works out peacefully so you can have a stress free holiday.