We are going to buy a new diesel truck to pull a 5th wheel weighing about 8,000 lb -- concerned with the new diesel and operating problems with engine. Have any of you noticed any difference or had any problems with engine??

 

Are you talking about the new fuel (ULSD) or the new engine, or both? I have a 07.5 6.7L diesel and as far as pulling capability, it does very well (30' fifth wheel). Other than the CEL being lit most the time the truck runs well and I'm pleased with it.

2007 Hitchhiker II 29.5 LKTG 5th wheel
2007.5 Dodge Ram 3500/6.7 CTD/SRW/SB/4X4/6-speed man/Big Horn Edition/16k Husky manual slider hitch

 

Ditto here. Except that I'd add not very good MPG. Tows and runs great, but I just had my first CEL at 2K miles...

It does what you need remarkably, just not too gracefull with the MPG and CEL issues.