Hi Chris
I went through a similar exercise when my wife and I started long distance travelling around Oz in our BT50 / slide on camper combo some years ago. After realising the slide on camper weighed 720kg empty (despite being told under 400kg by the manufacturer) we decided to go with a Pedders TrakRyder GVM upgrade. This included replacement shocks, coils and springs to increase GVM from 3.2 to 3.5 tonnes at $3349.00. We hoped this would easily cover the additional weight of a fully loaded camper. However, the weighbridge weight with myself on board, full fuel (130litres diesel), 90 litres water, 2 house 120amp AGM house batteries, empty fridge and freezer, esky, recovery gear, tool kits, compressor, poles for side awning, full 9kg gas cylinder and assorted odds and ends for bedding, cooking, clothes and fishing was 3.44 tonnes! This only left 60kg for wife, food and beer! What to do next? Ended up reducing payload by one battery and carrying less water on each trip.
My understanding from other 4WD forums is that insurers will not cover the vehicle in an accident, if deemed to be overloaded. They pick up all the broken pieces and take them to a weighbridge to check your weight after any accident. I hope that the fuel and water tanks are not full if we have an accident on an outback road as we will go close to our 3.5 tonne GVM limit on occasions.
On another note, Scottar is dead right about suspension modifications "voiding" warranty. I had paid for additional 2 years on the standard 5 year Mazda warranty with local servicing only to be told that they would not replace a front AC joint at 2 years and 26000 klm (apologies if this is not the correct term as I am not mechanical) because the suspension had been "lifted". This was not the case as I had carefully measured the chassis height before and after the upgrade and they were no different. However, I did upgrade to 265/70R16 Toyo AT tyres which were slightly bigger diameter than the standard Dunlop AT tyres that came with the vehicle. Despite my contacting Mazda head office and repeated explanations, they would not cover the AC joint replacement. Be warned!
By the way, my 6.2M alloy CC comes in at 1.7 tonnes fully loaded with fuel, ice and fishing gear, somewhat similar to your BC. Which means, I cannot tow the CC with the slide on camper on the tray. I justust have to manage with a swag on the tray.
I have been thinking of replacing the Mazda BT 50 but cannot find a suitable vehicle without spending the extra dollars on a GVM upgrade. I will be interested to hear how you go with the Navara. I know the standard Ram and Ford utes will do the trick, just not sure if this is the way to go into the future with fuel prices going up.
Cheers SS