80 series seems sluggish towing uphill from Tamworth

You don't indicate what seems sluggish, engine or auto.
" the car has never been to a workshop I simply change all oils, wheel bearings brake pads etc myself ", 363,000 is a long time without valve adjustment. What about ignition timing.
Start of by doing a simple compression test, see what that tells you,

That hill is probably one of the worst mothers you'll ever find.........long AND steep..

My 4wd went up to 116 deg towing a small van ( 1400kg ) up there a month ago when the outside temp was only 30 deg..

I find it's easier on the motor ( and temps ) to keep the engine revving fairly high, which in turn keeps the load on it down..

Cruiser,
Making an engine labour is a big mistake. That is the enemy of most engines.
At 363,000 on the clock it should still have a lot of life left in it especially as you service it regularly, also 10,000Km intervals is sufficient for servicing on those motors.

My 97 petrol 80 did the same thing on Moonbie so don't worry.
As Gronk says, "It is one mother of a hill"

Let the engine rev as those long stroked hard pulling engines like a few revs and in fact you will use less fuel by letting the engine work a little faster.
About 3000 to 3500 RPM is a good range when pulling up that hill and a little more wont hurt. They are red lined at a little over 4000 so doing 4 grand wont hurt for short periods.

The multivalve turbo diesel 80 series I had didn't like that hill either so don't worry. it was back to first also.

Not only was I back to first on that hill but on the subsequent sections of the Oxley Highway heading to Port Macquarie from Bendemeer I found the old girl to be sluggish as well.
Turns out that there is not as much oxygen the higher up you go so do not expect miracles when climbing hills and on top of the range. Let the engine rev more and don't push it as hard by making it labour.

Many of the guys around this area of the Mid North Coast often head up into the ranges with their trail bikes and also report considerably reduced performance when up there due to the higher altitude. That is very normal in the higher altitudes.

My V8 cruiser goes close to 1st gear as well but on that hill but I am pulling a heavier van now also.

Just as an example, I often climb the range west of Port Macquarie on the Oxley Highway heading to Tamworth and what I do is I select 3rd gear in the auto (5 speed) and leave it there till I have reached the very top of the climb. I use less fuel that way and the engine is in the right rev range to make it as easy as possible on the motor and drive train. The engine and gearbox decide if they need to go to a lower gear and often do.
I also stop for a cuppa at Stockyard Creek, about 2/3rds of the journey up the hill, and this gives the vehicle a break and the driver and passengers as well and that is such a lovely spot at Stockyard Creek.

Hope this helps.

Cheers, Bruce.

This 80 series...is it a country or a city truck.

Are you sure you had the right music playing....particularly comming out of Tamy.

cheers

Quick thing you can do is clean your MAF sensor. Makes a huge difference if its dirty