I had a spare tire stolen off my truck in San Francisco after going inside a Safeway to buy groceries. I have had a spare tire stolen off a boat trailer while going inside a Safeway in the Salinas area. I have had my truck broken into (side window smashed) while at a state beach parking area and my wife lost her purse, credit cards, and passport. I left a bike chained to a table at a campsite once and it was stolen while I was in town getting provisions.
It depends a great deal on where one travels and where one parks and the time of year as well as the type of vehicle. I would expect Class A motorhomes to be safer in this regard, especially with the basement storage areas to keep more out of sight, than a trailer or tow vehicle.