Is that 75 deg up along a wall or floor temperature right over the under tank heater?
They need a basking spot at least 85 deg (but not much if any over 90 deg) for at least a few hours a day to keep their immune system running well. Ideally you would like to have the air temperature in the 80's also but 75 might be close enough as long as it has a good hot spot to keep the snake’s actual temperature up. Assuming you have pretty cool background room temperature maybe you can insulate the back and ends of the cage to keep it warmer. I just bought a sheet of Styrofoam insulation at Home Depot that would work well for this, just keep it away from the heat sources.
I've not had a good humidity gage so I'm not sure what is right for humidity. I know the 10% in the house in the winter where I live isn't enough and the 30% - 40% in the summer doesn't seem to be enough either (mine do better in sweater boxes where I can keep it a little higher than the rest of the room). I live in Colorado and keeping humidity up is a constant battle for me.
I know you want to keep incubator humidity near 100% so I'm keeping a cage with a materially incubating mom pretty humid with sphagnum moss. I'll get a chance to take a good look at her in a couple weeks when the eggs hatch and see how she looks. I’m thinking as long as she is warm (she is) and not wet she shouldn’t have any problems with high humidity.
I think the main thing is that you not have it actually wet and particularly not wet and dirty.
I'm betting humidity is pretty high in their holes in Africa but I'm not sure. I did find a web page about northern Ghana going as low as 15% relative humidity December - February to 80% humidity in June to September. I found another page that said it averages 65% in the north and near 100% in the south. Apparently there is a slightly cooler and much drier wind off the desert to the north during what are our winter months in the northern hemisphere.
I'm not sure but I did find a study on ball pythons that mentioned the south so I’m thinking that is prime ball python habitat. Apparently even there it is a little drier in the winter. Maybe a LITTLE winter drying when the furnace is running and high humidity the rest of the year actually help to breed them in captivity.