We had intermittantly had an issue that seem to be tied to which way the wind was blowing and some combination of roof vents open. Put 2 of those on, one on each vent, no more issue! Sitting, moving, vents open or closed in any combination, and we have exceeded 65 by a bit a few times with no problem!
Ray, Cheryl & of course Miss Molly the four-legged child
the "stock" vent cover caps on most rigs are restrictive. How many home sewer vents do you see with "rain caps"?? The sewer smell will back up and fast with those caps. Dont believe me?? Plug a portapotty vent cap and come back after a few days. Plastic T or revolving sail fitting... I like inexpensive. YMMV