Yes it will keep the cab cold for two people and cool for someone sitting right behind the front seats but not the rest of the MH. If only two people you can put up a curtain behind the front seats to make it really cold up front.
My dash air keeps us cool until it gets above 95 outside but we do not have the heat of a engine working against it. Often poor AC on any vehicle is often caused by a bad hot-water shut off valve keeping the heater coil warm all the time. Easiest fix is a manual shutoff valve in the heater hose that you close in the summer.