Seems to me that replacing styrofoam with high density foam is worth considering.
Extruded polystyrene foam (XPS) also known as blue board or pink board comes in many different thicknesses and edge profiles. XPS has an R value of 4.5 to 5.0 per inch of thickness.
It’s reasonably priced, light weight and easy to use. This product is also used to insulate the outside of foundation walls and even under slabs.
Cost = This product is the middle of the road for these types of foam board insulation products.
Polyisocyanurate also known as polyiso is seen in all kinds of commercial building applications and more recently with residential building projects. Polyiso is typically used with a foil facing and it has an R value of 7.0 to 8.0 per inch of thickness. The reflective foil facing makes it an excellent insulation board when radiant heat is involved. The foil facing also makes it very easy to seal with good quality foil faced tapes.
Cost = Polyiso is the most expensive of the foam board insulation products however it’s the highest R value.
Polyurethane and polyisocyanurate are both closed-cell foams. They contain low-conductivity gases in the cells (usually one of the HCFC or CFC gases.) The higher R-Values (R 7.0 to 8.0) are the result of thermal resistance of the gases in the cells. This can lead to a couple of disadvantages including: off gassing of HCFC or CFC gases, and reduced R Value over time as the gas escapes.
I'm not sure if having a foil face is a problem or not. This is where I would do a bit more research. It seems to me there likely will be condensation on the inner surfaces in cold and or humid environments no matter what is used so the foil may be a do not care. If the foil is a does not matter then I would go with the highest r-value foam.
I think there are spray on foams that are very good and they do not have a foil backing. Maybe that is the way to go.
https://www.homeconstructionimprovement.com/foam-board-insulation-values/
See also:
https://www.rvingplanet.com/blog/ultimate-guide-to-rv-insulation/
I recommend that you focus on minimizing conductive heat transfer instead of radiative or convective heat transfer. Conduction is far and away the main heating/cooling process mechanism in our campers.
BTW:
Condensation occurs when water in gaseous form changes back into its liquid form. When warm air rises, or comes in contact with a cold surface it usually cools and loses its ability to 'hold' water.
The condensation collects on the cooler surfaces - often the ceiling and upper part of the inner walls of the camper because there is a lot of warm air with water vapor in it from your breathing and skin and/or cooking. As a consequence of the minimal insulation the side walls and ceiling of the camper they become cold enough (and rough enough) for the water to form droplets on the surface as it goes from gas back to liquid - i.e. condensation.