The simplest method:
Get these battery powered lights that run on 3 AA batteries. Instead of using the battery pack, connect them to the RV wiring using an adjustable voltage buck converter adjusted to 4.5 volts. Connect a switch in there if desired.
Check the voltage of the battery pack to make sure it's outputting 4.5V before connecting the lights. If you don't have multimeter, buying one will be well worth the investment for this project and any future projects. You'll need it to set the voltage of the buck converter as well. Final note, when doing any vehicle wiring, make sure you're connected to a proper fuse.
Pretty sure it's some fairy lights wrapped tight enough around the cat that they're disappearing into its fur.
https://www.amazon.com/dp/B07S3R6MKJ/ref=cm_sw_r_apan_glt_fabc_AWSXQR7DX6PYMNR1GNS0?_encoding=UTF8&psc=1 These on Amazon are perfect, they fit in the little tunnels and then the battery box fits on the opening on the bottom. I plan to hot glue them in place and then I also found the STL for a base if you're interested? https://cults3d.com/en/3d-model/home/christmas-village-base
It appears Lego Technic excavator is manually operated with a knob, have you considered automating that?
.
Consider DFPlayer for the sound https://www.dfrobot.com/product-1121.html
Record the sounds from some online source
https://bigsoundbank.com/detail-2149-excavator-displacement.html
.
Consider this for the arduino --> Adafruit Trinket M0 https://www.adafruit.com/product/3500
.
Alternate sound players
https://www.adafruit.com/product/2133
https://www.adafruit.com/?q=sound&sort=BestMatch
.
Consider "grain of rice" sized lights if they don't already exist on toy.
https://www.amazon.com/Ariceleo-Operated-Christmas-Centerpiece-Decoration/dp/B07S3R6MKJ
.
for the accelerator:
Look for springs that store energy in the twist/untwist action (like a wind-up musical toy)
use the spring to force the potentiometer back to "neutral" state.
use a ribbon wrapped around potentiometer shaft and connected to gas pedal to
turn potentiometer shaft (for acceleration)
Ariceleo Led Fairy Lights Battery... https://www.amazon.com/dp/B07S3R6MKJ?ref=ppx_pop_mob_ap_share
These simple fairy lights!! Super light weight so they won’t damage the plant 🥰🥰
How about some of these put wherever you need them?