As with the others, I don't take things out of cabinets or the fridge. I do put stuff back in the cabinets and fridge from counters, tabletops, etc. Driving conservatively and avoiding sudden moves helps a lot in keeping things where they belong.
In extreme cases (such as when going over bad transitions from a driveway to a road) where the RV sways a lot from side to side, it's possible for things to come out of my cabinets and end up on the floor. I try to avoid having breakable or messy things in upper cabinets; I use decent quality plastic plates and bowls and glasses, for example.
Watermelon is a bit tricky to carry well; the storage compartment underneath the dinette seat seems to work as well as anywhere (it keeps it from rolling around too much and getting enough momentum up to burst out of the cabinet door).