Why does our porta potty smell like ammonia in heat?

That sharp ammonia smell is urine breaking down in a tank that's past what the deodorizer can handle, and heat speeds it up a lot. Twenty five guys is mostly a urine load, and by midweek the charge in the holding tank is diluted to the point it isn't doing much. The chemical smell you're used to means the deodorizer is winning. Ammonia means it lost.

We had the same thing on a framing job last September. What fixed it for us:

1. Asked for a second service day each week while the afternoons stayed hot. That made the biggest difference by far.
2. Had the driver check the vent stack. Ours had a cracked cap and the unit was basically trapping the smell inside.
3. Turned the unit so the door and roof weren't baking in direct afternoon sun.

A stronger deodorizer dose helps a little, but it's a patch if the tank is just too full by day five. Also worth a look at OSHA 1926.51. Once you're at 20 or more workers you need a seat and a urinal per 40, so make sure that unit actually has its urinal and the crew is using it instead of the seat.