You're going to have to find out what the exterior surface is, so you can use an appropriate paint with the proper prep work, which might include sanding and will likely include degreasing to ensure that the paint sticks instead of any trace grease or oil (even from touching it with your hand) acting like a release agent. Cheap degreasers just dissolve the grease and spread it around, requiring repeated application. A good degreaser will actually remove the grease.
Keep in mind that for the base colour, lighter is cooler as it will absorb less heat.
I'm hoping that you can use something like the white elastomer roof coatings (around 3x thicker than paint) that are formulated both for water-proofing and for reflecting sunlight and heat away. Price can vary from ~$50 to over $200 for 5 gallons. Smaller cans available. Many manufacturers publish their reflection data so you can compare the performance of their different products, but some don't... A large portion in the price difference appears to be longevity, but you might just be paying for a longer warranty. The $80 for five gallon versions have practical reports just as good as the $200+ expensive stuff.
It goes on very thick. For his DIY trailer, prior to final assembly, FIGJAM applied this to pre-cut raw plywood panels laying on the ground. First spread using a 1/16th notched trowel, then spread evenly using a fuzzy (to keep it thick) roller with a long handle. In lower humidity, it dries in about an hour. Cured like it's 1/16th vinyl. If you spill a little and let it dry, it's like a sheet of rubber. He uses two coats and paints the edges too. It applied so well he didn't use any seam sealer when he assembled the panels of this trailer, but applied more of this paint to the seams instead. I've no reports yet on applying this to the sides of an existing RV of tradition RV materials – check the manufacturer for preparation and application method recommendations.
If you're wanting to add a touch of colur, Krylon spray cans stick to almost anything.
Jar Jar Sith Lord
Odd. No bears in the dump. Oh well, lets go across the road & pick blueberries.
[i]... but don't harm the red dragon that frequents the area from time to time. He and I have an agreement.