Tons of truth on this site, I haven't seen one falsehood anywhere.
My advice;
-The 5.3 with AFM (even in a 2018) is intolerable driving without a tune from Hypertech to eliminate the AFM. I buy new trucks every year and departed GM in 2016 as the 5.3 is sickening to drive even with a tune, low torque.
-Yes, towing MPG is horrible with all gas engines period. Expect 9-11. Even the Eco V-6's from ford are bad while towing, maybe worse than a V-8 in my experience. FYI, these are old trucks and both are bad at MPG but the Yo is much worse but has more power.
-Buy new if you can. Paying $18k for a 140k mileage truck is a rip-off and trouble for sure; ball joints, Pitman with all suspension parts etc., tranny (not that kind) etc. A new 50k MSRP crew truck is only 35-37k if you look diligently and don't mind a new leftover. Remember, buying a vehicle is not only about price up-front but cost per mile between day one purchase and the day you sell it (the true cost).
-No ridicule intended here but if you believe there may be a timing belt In a GM truck, I assume you will not be working one the vehicle. It has a simple chain and gears and has been around since the mid-50's.
Sincerely, Bear