If it's like our 2005 Cayman, you should have a stainless steel cap that fits over a steel nut. I forget the size, but it's not a common one used on most big rigs.
If you take a pair of pliers (best if padded) on one of the nut covers (like in you picture with a dimple on it), you should be able to wiggle it off. Then measure your nut width.
2005 Monaco Cayman 36PDQ
Toads: 2018 Equinox, 2008 HHR or 1993 S10 with ATV in bed