Do you mean modernize functionally or aesthetically?
Functional modernization might include a new power center,
solar panels,
LED lights, powered roof vent, new exterior fittings, pressure water system with new tank and clean-out hatch, front dinette conversion, higher lift
axle,
brakes, better stabilizers, and even high-end stuff like lithium batteries, inverter, compressor fridge…
Aesthetic modernization could mean any number of things: new countertop, sink, cooktop (recessed, maybe), cabinet hardware, window coverings, dinette table… even simple things like replacing yellowed plastic moldings on cabinet edges and Ensolite seams. Some have polished aluminum window frames and door hinges to a bright finish.
Depends on what your idea of “modern” is and what looks “old” to you now.