Depression or dementia?

I'm new to this particular forum, and looking for some perspective. My husband is currently undergoing treatment for recurrent esophageal cancer and that's the board I'm active on, but I'm here with questions about my mother and possible dementia and depression symptoms. My parents are both 86 and in good physical health. They live in their home about 5 minutes from both my sister and I. My dad still does all the outside chores like lawn mowing, snow blowing, etc. He scoffs at our suggestions about hiring help because I think it's his only outlet and he refuses to give it up. He also does all the cleaning, laundry, pretty much everything but cooking. They eat out many times a week and my mother will occasionally make dinner. She will go to the store and do some errands in the mornings, but then takes a 3 hour nap every afternoon. This isn't a new thing, even when we were kids, she would nap all afternoon and be really put out when she'd have to get up to make our dinner.
She is incredibly negative, hateful when talking about pretty much anything or anyone, not a new thing. She's focused on thinking something, anything, is physically wrong with her, has seen every dr from all different specialties, seemingly hopeful that she will have some health problem uncovered. She claims the doctors "poo-poo" her endless complaints and unexplained imagined illnesses. She has always been like this. She sees a different doctor nearly every other week with new, unrelated and unfounded symptoms. Never satisfied or grateful being told nothing is wrong with her. Whenever anyone comes down with a real illness, friends with falls, cancer, etc, she immediately starts complaining about her "problems", trying to put attention back on herself and her imaginary illnesses.
She literally spews mean comments and the negativity is relentless. There is no talking to my dad about our concerns that maybe she is depressed...to them, any thought of mental illness is a weakness and therefore unacceptable to them. I believe with all I know that she has always been depressed, but nope, it's got to be something physical that just hasn't been discovered.
She is always on the iPad, signing up for sweepstakes and coupons for items they don't need, etc. Last week she texted my sister to tell her they won a prize for $16,000 and that she put my sister's phone number down as the contact number for the people to call with details! This is getting crazier as time goes by. My sister had a heated conversation with her yesterday about scams and how she should not be giving out any information to anyone but my mother just became angry and told her she wouldn't be sharing any of the money with her...whaaat? My sister had a talk with my dad about her behavior. He's concerned as well, but will NOT disagree with my mother or go against her in any way. We think he covers up a lot of what else may be going on with her. We are at a loss as to what to do.
Oh my goodness, what a rant. If you've read this far, I thank you! My mother and I have never had a good relationship. I visit every week for a couple hours and we talk on the phone several times during the week, for me out of obligation only. I truly love my dad which keeps me feeling like in order to see/talk to him, I have to put up with her. It's taking a toll. I've got enough on my plate already.
Does this sound like dementia? This has always been her personality so it's hard to tell, but I think it's time to figure it out. None of us want to deal with the wrath that would come with trying to talk to her about this. Any suggestions would be appreciated. So sorry for this long post.