Blow by under heavy load when towing....why?

[MojaveJoe]

Driving a 2004.5 Cummins 3500 - 600 motor, w/ 160K miles.
Last summer when towing my 13K fifth wheel I ended a 3000 trip with blow by oil half way back on the head cover. When driving around town or not under load get no blow by.
Did the upside down oil filler cap test. No unusual/ noticeable movement. Several times during my trip I was pulling hills at 70mph, 2500rpm and 1400 EGTs. Im thinking this extreme loading for 5-10 minutes was causing the blow by. No extreme oil consumption.

Hoping Im not looking at a rebuild.
Any and all thoughts and experiences welcome.

 

[Ozymandias]

If you have oil on top of the cylinder head then it is as simple the plumbing under the plastic cover is deteriorated.
I had the same problem, installed all new rubber hoses and no oil there anymore.

Edit; you would see the oil down at the front axle and at the side of transmission with a intact breather assembly and excessive blow by.