Eating out and finding convenient food isn't particularly expensive in Japan (I've always found it to be considerably cheaper than London, even when the yen was sky high against the pound and the Euro)

The trick isn't eating and feeling full on a budget, it's eating well.

It can be done, and there's help for the asking available on this forum. For the sake of the price of a few apples and pears there's no point suffering, locked away for hours or days, in a hotel toilet.