You may have several problems. Check all the battery cables, including the cables that connect the batteries to each other and the main cable to your converter. Often they corrode inside the casing and will test just fine when checking voltage or when testing resistance, but they will not be able to handle any load. Best way to test them is to tug and bend them, if there is little to no resistance as you would expect with intact wiring, change them out.
You also need to either drive faster, get a better tow vehicle or get a better GPS. St Louis to the Kansas line is only about 250 miles. Really shouldn't take you 9 hours Good luck with your problem.