Well not being experience with the Itasca or how the MPH guage works I would have to guess. The first thing I would do is determin if it was electrial or chain driven. If electric I would need a wiring diagram. If mechanical I would ck the chain,gears etc. MPG I am assuming you are talking speedometer? Does the odometer work? Are there any other guages not working? Welcome to the forum
Guessing it works through a computer so I would start checking at wherever the sending sensor is located. Not much help but maybe keeping it current someone with experience in this can help.
it might be worth contacting them direct, i'm assuming it's a winnebago
i bet they have a technical dept
do you have a satnav, use that instead
i never look at the speedo, only the satnav