Hi,
I used to live in Northern Alberta, so I'm used to fighting car seats into all sorts of trucks.
Try this...
It'll take both of you to do this.
Shorten the tether strap to where you think it needs to be. It might take you a few tries to get it just right, so don't aim for perfection the first time.
When you think you have it about right, have your Dh hold the MA up in the air while you feed the tether strap through the fabric loop and over the middle anchor. Once it's clipped on, have Dh lower the MA onto the vehicle seat. Fiddle with the tether strap webbing until it's sorted out -- it might be sort of messy/tangled a bit at this point.
Once Dh has lowered the MA onto the vehicle seat, check to see how taught (as in, tight) the tether strap is. You want it to be fairly taught before you continue installation, but not so tight that the MA is hanging in the air.
Once you've determined that the tether strap is the correct taughtness/tightness, carry on with installation via seatbelt or LATCH.
(Old school techs will be reminded of this technique as it dates back to when tether straps were a beast to adjust [H-clips], and it really was trial and error to get it just right)
Anyway, try this method and see if it helps. I know the V-shaped tether is a lot to deal with in trucks. Lots of webbing, big adjuster mechanism, small space, so close to the glass, and just sort of an odd undertaking to begin with.
HTH's