I agree with Angela, purley on the comfort factor. I travelled 3 hours by car to my parents on the weekend between Xmas & New Year, I was 35 weeks, and it wasn't pleasant. But my baby has it's butt under my ribs which causes a lot of pain as soon as I get in the car. I wouldn't go much later than 34 weeks.
As for your dad (((((((((((((huggles)))))))))))))))) I didn't respond in your other thread as I had no idea what to say, I apologise for that. My mum is disabled and altho much different illness, I can understand how difficult visiting is. As much as you long to go see them, it's so emotionally draining to see them poorly. As soon as I get there, I long to get home.... it's kinda like "out of sight, out of mind", i.e. if you don't see what the illness is doing, it's a bit easier to cope? Make sense?
I'm so sorry he is poorly

I too say, if you feel up for it, then go. Explain to DH you will need him for emotional support as it will be upsetting for you to see him like this.