David,
Yes, we need more info, but on the FWIW category, I did the look on a single engine Mainship 400 in 2018. Anchored just about half time. Total fuel for a 6000 mi trip was $7,774 for fuel (averaged pretty close to $2 a gallon) and $5,520, fuel clearly being the biggest expense. On occasion, I traveled along with a Mainship 400 twin engine whose fuel consumption was virtually the same.
I now have a Mainship 430 and the fuel bill with twins isn't appreciably greater than the MS 400 single, when operated at hull speed. The twins offer a lot more flexibility and much easier maneuvering. Maintenance is not double, but is more. However, the 400 single was a great boat and no regrets in owning it. If you're selling at the end of your Loop trip, resale will be much more important.
And you'll unlikely do much of the Mississippi... just St. Louis to the Ohio.