It is 1-1/4", and be careful how much you add, due to backpressure. The best option is a "Genturi-Type" device that routes the exhaust above the roof without adding backpressure.
I built my own using lightweight 3" PVC, a 1-1/4" EMT elbow, some aluminum strips, and 2" suction cups to hold it onto the side of the MoHo.
Cost was about $15, labor about an hour.
2005 Georgetown 342DS XL, Full Banks Package
2005 Liberty Renegade 4X Toad
1989 Wrangler Sahara Toad
1984 19.5' Beachcraft i/o 5.o liter, 4-bbl (and you thought your Moho sucked gas!)
D/W Carole and Maggie the Teacup Lab (Black Chihuahua)