RV sub panel on main feed
Yes, you can tap into the conductors in the cabinet.  As long as it meets...Chapter 3,  Article 312  cabinets and cutout boxes. ( Installation 312.8 )  Over current enclosures taps,and feed through conductors 75% fill. And taps may run up to 25 feet..240.21  wire is a least 1/3 the size of the conductor. your tapping into