Matagorda! We're almost neighbors.
40 miles south of Houston here.
As far as the insulation goes, you'll be glad you did if you spend just one day/night in the bus. At the very least, consider insulating the floor, walls and ceiling with 1 inch polyiso/blue/pink from Home Depot/Lowes. More is better. Install a vapor barrier too. The vapor barrier is important. 4 mil plastic sheeting under the insulation works very well.
The biggest heat gain/loss is the roof. Even painted gloss white (which helps!), it cooks the interior quickly. And at night it radiates heat like the tin can that it is. Very uncomfortable in hot/humid weather.
Let me give you a point of reference as to heat transfer: my 38 foot BB has a bulkhead halfway down the bus to separate the cargo area (rear) from the living area (front). The AC is a new 15k btu rooftop heat pump installed in the front half. I have at least 1'inch of insulation in addition to the factory insulation on all the walls/floor/ceiling and at high noon 15k just barely keeps up. And at night it will freeze the balls off a brass monkey.
If all you plan on doing is sleeping in it at night, consider getting a unit that also does dehumidification (like mine). You can easily sleep in a bus at 45% RH and 85F, but you will NOT be able to sleep in a bus that is 85% and 85F. Been there/done that/aint doin' it again! Far better to just sleep in a tent with a little breeze than to baste in your own sweat.
And BTW: welcome!