Engine light on in 1st gear

Thank you everyone. It does not appear to be a brake issue because there was no additional slowing of the rv when I came to a stop. It felt normal in that respect and after driving 3 miles to the dump location and back, there was no odor of hot breaks. Slipping transmission in first gear could be the problem. I let it idle while I emptied the tanks and filled the fresh water tank. Engine temp remained normal (189-190) for 15 minutes. Then, when we left the dump location, it overheated again, in 1st gear and immediately cooled down to normal, in 2nd and 3rd and 4th gear. I'm taking it to a local auto/transmission service location tomorrow morning. They don't normally work on rv's but they work on large trucks, maybe they can diagnose the problem. Freighliner in Minneapolis (closest to where I am) said it would be $350 to scan it for error codes. they said that the error code might only say that the engine overheated, not what caused it. I can't afford that kind of help.

you need to find out what code was logged when the check engine light came on, could be a sticking egr valve or a sticking variable geometry turbo but until you verify the code you are just guessing. 204 degrees won't bring on a check engine light

Are the engine unusually rpm's high while the road speed is so low when in first gear? This would be a symptom of the transmission slipping.

Shift it to neutral while it is laboring and see if it will coast properly.
Brakes my not be releasing.

Do you have a brake dragging??