Both TV's were bought stock and used (I was a mechanic for about 20 years, so I was thorough in the "check out").
TV#1, bought in 9/2009 for $15,000: 2007 F350 XLT Ext Cab DRW 4x4 6.0L with 148k miles. Used it for 4.5 years and traded it in at 182K miles. Unit was trouble free in the engine. 9mpg towing RV (11klbs 36' TT) and about 14mpg hwy unloaded.
TV#2, bought in 6/2014 for $31,000 (less trade of TV #1): 2011 F350 Lariat (loaded) Crew Cab DRW 4x4 6.7L with 112k miles. Currently in use and has 114k+ miles. Unit is MUCH better for towing. 10mpg towing RV (14Klbs 40' 5er) and about 16mpg hwy unloaded.
Personally, I prefer buying good, stock, used vehicles for towing. They are *SO* expensive new. The diesel's generally can be expected to provide as much as double the life of a gas counterpart. Maintenance is the key. I always buy and configure for more than is needed (performance / capacity wise).
Good luck with your decision. But is your current TV is doing the job and is reliable, 155K miles on a well maintained gasser, that is currently in good condition, should be fine. For the same on a diesel...
Tina + Dutch and Bell, Bobo, Harley, Buddy (the dogs)
2011 F350 Lariat 4x4 DRW SuperCrew 6.7L (B&W setup w/Air Lift 5000 Ultimate)
2014 Columbus 365RL (Airflex and Voyager package)
Nights Camped 2014 - 154
, 2015 - 32, 2016 - 4