Exoterra tank and temperature issues

I have been having a hard time keeping my ball python's tank the right temperature. Currently it is an exoterra 18x36x12 tank with a zoo med heat under tank heater. Humidity at 55.... working on keeping it the right level with some tinfoil over the top screens. I have a zoomed thermostat but it just has a dial, no temperaure gauge, to keep the UTH on high or low. The junky thermometer I got from Petco says it is 65 F right now, but it is much warmer inside the hides.This has been happening since me and the roommates haven't been using the heat as much as it was getting too expensive. Should I get another UTH, or a heat lamp? How do I improve the temperature?

I get good results with lamps only in a tank. One red for night (my house gets cold), white for day. A lot of people believe lamps aren't necessary, but they do work. I tried UTH on the tank, but my Spider wouldn't go into the warm hide with it on.

A uth provides a hot spot only and does basically nothing for tank temps, if using a uth the room temps have to provide the ambients for the tank othetwise you need to use another heat source like a light, portable heater or a rhp.... You also dont need to swap your lights for day and night especailly to a bright white bulb, the red can and should be used 24/7 since balls dont like bright lights and cant see red, but better then a actuall light is a ceramic heat emitter. But do note if using a light you will need to tale precautions to add more humidity to the wnclosure as heat lights will dry up humidity.

Also how are you measuring what temps the uth is getting to, if you cantnactually measure the uth's temps you should turn it off till you can so you dontnaccidentally burn the snake and what are you using to measure ambients... If its one of those dial guages know your temps are more then likly not what its saying they are, you want a digital with a probe or a ir heat gun

Go to Walmart and buy two accurate thermometer/hygrometer's. They are digital 2-in-1 devices for like $10 each. Put one on the warm side, one on the cool side. That will give you better humidity and temperature readings. Your UTH NEEDS to be on a thermostat. Period. If you don't have one, you can get one on Amazon for about $40, or if you have the money, invest in a Herpstat. It's $100, but well worth it. To keep ambient temperature up in a cage that size, you either need to heat the room, invest in a 100w CHE (ceramic heat emitter), or an RHP (radiant heat panel). You'd have to modify your lid a little for the RHP, but it would help.

I also use heat lamps to maintain the ambient temperature in my tanks during the day and at night.