How do I darken the stain on my solid wood cabinets & walls in my RV?

If you want to stain you can use gel stain over stain with little prep.

https://www.minwax.com/how-to-finish-wood/change-stain-color-with-polyshades/

Or you can paint.

How to Paint Cabinets

Make sure they are clean and dry. Remove the doors and hardware. Mark the doors and cabinets with tape where they go. Lightly sand the doors and cabinets to remove any gloss and roughen the surface for paint with 220 grit sandpaper or a green Scotch Brite pad.. Use a tack cloth or damp rag to remove dust after sanding. Prime with a stain blocking primer like KILZ. Acrylic or water base paints are low-fume and clean up easily with water. Alkyd or oil-based paints require good ventilation because the paint contains solvents that can irritate your lungs and make you feel sick. Alkyd options require mineral spirits for cleanup, but they provide a hard, durable paint finish. Whichever you use, buy the best-quality paint you can afford for a lasting kitchen cabinet finish. Seal with at least three coats with a water based polyurethane. Use a small foam roller and foam brush for a smooth finish.

Unless they are sealed, you can scuff with steel wool then clean with denatured alcohol and you are ready to go. If sealed, you will need to degloss then proceed.

If you have repairs to make, do them first.