gmc 2500 6.0 eng

YA I have a 2000 Silverado 2500 with the 6.ol, 230,000 miles on the engine, very reliable engine, my Year was the last with the Cast Iron Heads, I pull a 4000lb Lance 1685.. Engine feels like its finally getting a little tired going up hills, but I have 4:56 rear end gears because of my 33 inch tries, tows in Drive good on level ground. But definatly thirsty, gets 11mpg around town with no trailer, 7-9 with..