It is crazy to try and diagnose without being there but I’ll bite.
I have had surging in several situations:
Being in 3rd gear, cruse-control and going down hill. The truck trans is slowing it down and the cruse is speeding it up
Braking with the brake controller up to high. The trailer will slow harder than the truck and turn/brake on and off.
Could even be a head wind issue, its amazing how much drag can come from a trailer.
Then ,in my experience, there is a TV issue and should get an “engine light”
2011 Wildwood T26BHXL
2004 Yukon XL Denali
43yrs of camping; one lucky guy