When parent doesn’t care how hot it gets, any advice?

If your FIL depends on you for food, laundry and dog care, his mental capacity may not be too great.

If he has a doctor and he finds out about him living like this, he may send a case worker out. If he doesn't comply with her, she may petition that he can not live alone.

What do you think is behind his behaviour?

Is he putting the heating up too high? Or is it summer & very hot where you live?

Is he hot but too proud to ask to come into your house? Is he welcome to anytime or not really?

Would he like to be cooler but can't afford portable air-con?

What about buying a box fan & suggesting the old way of a wet towel in front of the fan? It would be cheaper & HIS decision when to put the fan.

My father keeps his a/c set to 86 in the summer. He says he’s trying to save money. I will lower it to 80 or 82 and he says it’s too cold for him. He is anemic, but I still feel 86 is too hot. Time after time I tried to reason with him but couldn’t. After a while, I gave up fighting over the thermostat since he would just turn it back up.

Is there a way to disable the heating unit during the summer months?

You can install wifi thermostats that are controlled by a phone app. That way you could set the temperature in his place at, say, 80° to keep him coolish but not boiling hot. If he jacks the temp up, you can bump it back down.

Your father is in grave danger. I believe that you show yourself to be aware of this fact. Lately a few posts on Forum make me think that we have slipped over the "You can't make this stuff up" line, and I fear that some of the posts ARE being "made up".
It takes no time at all for an elder in a trailer to lose consciousness. I have a friend in an old historic park in Palm Springs, and ever year there is an accidental death, one at least from an old unit going out of commission and not fixed quickly enough. Elders are poor at recognizing heat stroke and almost always enter hospital ERs dehydrated due to having lost the drive to take in fluids that younger people still have.
I do not know if you could be held legally responsible. Certainly it is my own opinion that you are morally responsible.

Older folks can't keep themselves as warm or cool as younger people can, and they don't move around as much either.

Any room with a temp of 99 degrees is too hot for anyone, especially an older person.    Since he's not particularly cooperative, I would establish a checkup schedule, say every half hour or hour, just to make sure he's safe.  

I wouldn't gamble on getting in trouble legally; even if charges weren't pressed, I'm guessing someone, somewhere would contact a news outlet and you'd be publicly shamed.

And the fact that he lives in a trailer on your property would be a major negative factor against you.

Yes, he has a right to kill himself. What I would worry about is dehydration. He may have stuff to drink but does he. And in 99 heat it should be water or Gatorade. Seniors dehydrate very easily.

A/C makes me cold. We keep ours at 77° just enough to get rid of the humidity and be comfortable. I was always told that trailers are very well insulated. Maybe he gets cold with A/C. Does the trailer have ceiling fans? You can set a/c high and get a floor fan that can oscillate to help distribute the cooler air.

Try not to boss him around. As my daughter says, make them feel they made the decision. Ex daughter gave: resident needed a bath and refused it. Daughter said "but Mrs. S wouldn't you feel so much better all clean and smelly good in nice fresh clothes" Resident "yes I would" and goes with my daughter.

IDK, it seems to me that a lot of stubborn, "independent" seniors do die every time there is a heat wave , perhaps because their bodies ca no longer recognize they are in dangerous territory. I can't say whether or not the authorities would construe that as criminal negligence on your part or not, but public opinion might and besides, I'm sure you wouldn't want that on your conscience. At the very least you can try to provide him with the means to cool his trailer (window A/C, fans, hopefully an appealing shady spot outside) and plenty of opportunity to drink lots of cool beverages, even if he refuses to use them.

Why not a window a/c? At his age, heat stroke is a possibilty, 99 deg is way too hot. Have you been appointed his guardian? If so, his death in this case could be considered neglect on your part.

I'd say its unlikely, but I suppose there is a possibility if you are considered responsible for his well being. It seems to me there is always a gray area when it comes to situations involving elder abuse and neglect.