My senior dog struggles to breathe. Should I consider euthanasia?

Poor Xica! I'm so sorry that she's sick, and that you're in the position of having to consider what is best for her right now. This is always a difficult situation.

Her x-rays do appear to show very severe pleural effusion. As I'm sure your veterinarian has probably discussed with you, pleural effusion of the type that you have described (bloody fluid) is almost always caused by one of two things: congestive heart failure, or cancer. The amount of fluid in her chest makes it hard to get a good look at the size and shape of her heart, or to determine if there are any obvious tumors in her chest, so it's hard to say which of these possibilities is more likely without additional testing. But unfortunately, even with aggressive treatment, neither of them have a good long-term prognosis.

Knowing this, if I were in your position, I believe I would probably decide to let her go rather than pursuing hospitalization, chest tube placement, etc. - these are invasive, uncomfortable procedures that are unlikely to make a significant difference in the outcome for her, because of the severity of her underlying disease.

That being said, the decision of when to consider euthanasia is a very personal choice, and one that is different for every pet owner. If you feel that you need to do everything possible, and want to pursue advanced diagnostics to try and get a more specific answer as to what's wrong, there is nothing wrong with this at all - but if you were my client, I would try to gently caution you that her underlying condition will almost certainly be fatal regardless of what you do.

I hope you're able to find peace in your decision, whatever it might be.