[crusierdad76]

when you say complete tune up If the plugs and wires are already done and the air filter is good there isnt a lot left to tune up Like Crewzin said a little more air in the tires try to keep it at a steady RMP level probably under 2000 no quick jack rabbit starts
Guess you could do a oil change but that isnt going to help the MPG's I dont get real good MPG's but i do get close to 22 I have to think if he is getting 10 there has to be something that is wrong big time

One more thing to think about IF the fuel filter is clogged it wont really get bad millage it will be starved for fuel and run like crap probably bucking and spitting and for sure pulling down on hills

maybe cleaning the air intake running some lucus fuel additive through it maybe a couple bottles at this point any thing should help