こんな感じかな？と言うイメージ

wip.rb

# frozen_string_literal: true

def build_total(reports)
  [reports]
end

def original
  campaigns = Campaign.where(id: 1..3)
  reports = MediaDailyReport.limit(3).to_a

  campaigns.each_with_object({}) do |campaign, h|
    reports
    h[campaign.id] = {
      campaign_name: campaign.name,
      reports: reports,
      total: build_total(reports)
    }
  end
end

def wip
  campaigns = Campaign.where(id: 1..3)
  reports = MediaDailyReport.limit(3).to_a

  rows = campaigns.map do |campaign|
    attributes = {
      campaign_name: campaign.name,
      reports: reports,
      total: build_total(reports)
    }
    [campaign.id, attributes]
  end
  rows.to_h
end


def main
  ap original
  ap wip
  ap original == wip
end

main

実行結果も貼り付けたかったが、フィルターデータでは無いので・・・すみません。