What fluids to service in used RV?

Check the house box converter charger and house battery(s)including cleaning of all battery connectors, electrolyte levels and DC voltage. With rig on shore power, voltage should be around 13.6 volts, with engine or generator running around 14 volts, disconnected from shore power for some time etc., voltage should be around 12.6 volts. It is good to have battery load tested if it is not fairly new, for reliable power. Older converter chargers can overcharge batteries and boil off electrolyte. Might want to consider updating converter charger if old type. Check fridge for proper cooling in electric, propane and auto modes. Run generator, let it warm up and run roof AC with microwave same time to test proper operation. Set up awning, inspect, and stow it carefully. Might want to install awning lock device to prevent accidental unfurling while underway. Make sure tires are not older than 5 years old by date codes and with no sidewall cracking. If you hear any steady thumping while driving, check tires for lumps where tread is separating. Watch for any pulling of front brakes, change front brake flex lines if not newish when having brake fluid flushed and replaced and brake system serviced as needed.
Tires and brakes are very important in RV's for safe driving.