The old saying “measure twice and cut once” perfectly fits in this situation. It is best if you measure more than twice in this case because you will cause thousands of dollars’ worth of damage if you make one wrong move.
RV manufacturers all use the same sizing guidelines for skylights, which makes purchasing a new one fairly easy. There are only a few different sizes available, so it shouldn't be difficult to locate the correct size for your RV.
Climb up on top of the RV and measure the length and width of the base of the skylight.