Can I swap out the 3 speed transmission for a 4 speed?

My 1989 chevy 30 van chassis with class c camper on it has a 350 eng. 3 spd auto tranny seems like it is very high rpm at 55 mph and bad gas mileage too, can I swap out the 3 spd tranny for a 4 spd and get lower rpm and better mpg? And what tranny would fit? Thank you