While on the road, there are a few ways to eat. Japan is known for its amazing food specialties.
Cooking
The most comfortable way to feel at home while on the road is to cook for yourself. Buy fresh food in supermarkets and cook it in your camper.
The easiest way to cook in a camper is by using a portable stove and disposable gas cassette:
If you like to eat rice and have electricity in your camper, you can also get a tiny portable rice cooker:
At campgrounds, you can enjoy BBQs. Some campgrounds have fixed BBQ setups (they are often not located directly at the campsite and are all grouped in a cooking area), but you can also carry a portable BBQ with you.
NOTE: In Japan, it is illegal to carry a knife with blade over 6cm (about 2.36 inches) in public without a valid reason. While camping is a valid reason it is recommended to not show knives too much.
Supermarket Bento and Delicatessen
If you're too lazy to cook or your camper setup is tiny, this is a viable option for short trips. Japanese supermarkets always sell bentos and small portions of cooked food: salads, fried chicken, sushi, and more.
Restaurants
Eating at restaurants in Japan lets you enjoy local specialties.
NOTE: In the countryside, it might be hard to find a restaurant at night in Japan. There are few options and most close early.