First, a whole lot of things can be classified as 'campaigning".
We used to have a local candidate who regularly ran, got his State Campaign Funds, and used about 90% of them to hire his wife as campaign manager and his 2 kids as campaign staff. Nothing illegal about that – a candidate should be able to hire whomever they wish. And you can probably count on family members being sincerely interested in your campaign. He lost regularly, but had a good family income from it.
Candidates have been known to use this money to rent a campaign office in a building owned by a friend or relative. And that’s reasonable; most commercial landlords aren’t particularly interested in a tenant who needs the space immediately, but probably only wants it until the election, and (as a campaign committee) has no credit history.
It’s not unusual for modern campaigns to pay for a cell phone for the candidate & primary staffers. Paying for transportation, hotel rooms, meals, etc. is quite common. I’ve even seen campaign funds used to buy new clothes for a candidate. All reasonable ‘campaigning’ expenses. But this can also be used by the candidate & friends to live ‘high on the hog’ during the campaign. (Especially if you don’t have any real expectation of winning.)
I’ve known of campaigns that bought a new vehicle for the candidate to use in traveling, then after the election, sold it to the candidate (as a used, depreciated vehicle, at considerably less than the campaign paid for it.) Same thing can be done for computers, fax machines, copiers, and other office equipment. All of them have a greatly reduced value as ‘used’ machines, even if it was only a few months of campaign use.
Second, who would we trust ‘check’ on this spending?
What & where money is being spent is very confidential information for a campaign committee. They would object very strongly to having this ‘checked’ by someone who could easily be supporting an opponent, and could pass the info along to them. I’ve managed campaigns here in Minnesota, where we have public disclosure laws that require reporting of campaign spending. We carefully study the opponents reports to try to figure out information about his campaign strategy.