The a/c should already have a temperature control built in (assuming it is not on an external thermostat). If it's not working, need to find out why and fix it.
Insulating what? The entire RV? Some easy things to do are to put out the awning for shade (reduces heat gain on one side), put pillow-type insulators in the ceiling vent(s), add a reflective cover under the shower skylight if you have one, and use drapes or shades on the windows. Reflective window film helps too. Is the roof nice and white? That helps reflect sun heat.