i don't know if there is a good answer to this. A lot of small business owners make less than they could working for someone else, but they like doing their own thing, so that makes it worthwhile to them.
I knew a couple that owned a small business and worked 60 and 70 hour weeks for a net of less than $50k a year. Both were well educated and hard workers, and I doubt either would have had any real trouble finding a 40 hour a week job making close to what their combined income was with both of them working 60 hours a week. And they would have had company paid benefits to boot.
I second getting a good small business accountant. Paying off your trucks may make you feel better, but it may not be the best thing to do financially. Putting the money in the bank might be a better answer. What you may really be doing is spending your profits on capital investments and not even realizing it. Your profits may be a lot more than you know if a lot of the money is going back into the business. The accountant can help you get a good feel for where you really are financially, and then you can decide what makes you comfortable. Personally, I would not be a big fan of paying off the trucks. yes, there are some minor benefits if you run into some slow times, but taking that money and putting it in the bank is also a good way to prepare for the leaner times, and there is a lot more flexibility with cash in the bank, than a paid off truck sitting idle.