Have you actually measured the voltage at the two wires supplying power to the TM? That boat cold have any of many wiring issues that result in not getting 24V at the motor - even though you think you have wired for 24V at the rear. Also, what gauge wire is the TM harness built with? Undersize wire, poor connections, or unknown splices can readily cause issues. Minn Kota and Motorguide build very similar competitive models. There's usually an external reason for wide discrepancies such as you speak of.
I will check it today with a voltage meter. I hooked it up the same way I had the motorguide hooked up. I'm also using the same wiring. I just connected the new TM to the existing batteries/wires. But I will try the voltage meter.