Yup basically just seeing if the t-stat is limiting the power but regardless, i think an 88 watt panel is too weak for a 2 foot high cage unless you keep the room fairly warm.
And the slate idea is what i mentioned earlier about putting something under the RHP for it to actually heat as RHPs dont create the hot spot themselves like flexwatt, they need an object to warm up. You can also just use a large hide as well. If you are using RBI black ones, they soak up heat like no ones business. I used those when i ran RHPs and they would get nice and toasty warm.