The average length for a shank is 13.75 inches. My Reese was the optional 17.75 inch shank. It worked just fine, but since my new camper has a higher tongue weight and I no longer tow with a Jeep, I went ahead and cut it down to 13.75 inches. I did use it once to pull my new TT, and didn't notice any bending of the shank with the higher tongue weight (only about 720 pounds), but, moving the hitch head out away from the receiver lengthens the hitch assembly, and moves the weight of the tongue further away from the axle. This increases the leverage the tongue weight has on the axle, and increases the moment of force exerted on the TV suspension.
With an extension, you'd see the same sort of thing. Add to that the weight of a golf cart in the bed of the truck, and you are going to need some serious weight distribution action.
2014 Coachmen Catalina 253RKS
2016 Keystone Summerland Mini 1750RD
2012 F250 Lariat 6.2L Crew Cab FX4
Reese dual cam SC/WDH