Two points for you:
If it didn't come with a ladder, it probably does not have the necessary backing to add a ladder.
But, to employ an old carpenter trick, you can map the framing by using your middle finger to tap repeatedly across the target area. When the pitch goes up, you have framing and when it goes down, you have hollow. This works most everywhere including houses. Do this laterally (for studs) and vertically (for blocking) to map the area. As I said, you probably will not find the necessary blocking but you might get lucky and find the stud spacing sufficient for a ladder to be added.
2004 Damon Escaper 4194 LX400
Freightliner XC Chassis,
Cummins ISL 400hp