OP - I haven't seen a coach like yours, but it seems to me that if you have only a 4" step up from cockpit to house:
1. A four-foot ladder won't serve, you'd need a five-foot or a cut-down six-foot, which would give you wider steps but take up more room.
2. You have a lower "loft" than a coach with a higher step up, say 12". We rented a C with a high loft and it was possible to sit up in.
The C we started with had a very low loft and was a high climb AND hard to do things like make up the bed because there was very little headroom. Admittedly, the residential mattress we used cost us a few inches there.
I like the plan detailed above, starting with a wooden ladder, modifying it, securing in place, and adding grip handles.
Still, if you find you can't manage, comfortably, safely, please don't force it. Can't have a major injury doing something that's supposed to be fun.
