You can find an explanation here:
Suffice to say the higher the R Value the more resistance there is to heat transfer. For a frame of reference, in houses, R15 walls are common except in northern parts of the country where R19 or R21 is better.
Different materials have different R Values per inch. So an inch of fiberglass has an R value of about 3.5, but an inch of white styrofoam (beadborad) has an R value of 4 and an inch of extruded polystyrene (like DOW blueboard) has an R value of 5. Wood only has an R value of 1.25 per inch and aluminum much less.
How the truck camper manufacturer gets to an R 6 and R 12 depends on the insulation material and its thickness. Two inches of extruded polystyrene would be R10 while it would take about 3 inches of fiberglass to have an R10.
For energy ratings of homes, they take into account the R value of everything: the structural members (studs, rafters, etc), windows, the plywood, sheathing, siding, drywall, etc. So the true R Value of a wall takes into account everything, not just the insulation. So a house might have R19 insulation in the wall but the AVERAGE R value of the entire might only be R 12.
In truck campers, the area that structural members take up in the wall cause cold spots, just like studs or rafters in a house. So wood structural members are better – warmer – (higher R Value) than aluminum. And molded fiberglass campers have very few structural members and are warmer yet.
The truck camper manufacturer just tells you the R Value of the insulation. They do not tell you what the average R value of the entire wall or roof assembly is, because that would be so much less when you figure in structural members, windows, etc.
But yes, all other things being equal, the higher the R Value for the given thickness of the insulation the better.
For retrofitting a camper you will have to research the 3 types of heat loss: conduction, convection and radiant. Be aware that radiant heat loss is very minimal relative to the other two types so don’t get sucked into foil faced insulation products touting their heat reflective qualities.