I recently bought new sunraysia wheels and toyo light truck tyres for my tandem trailer (it had used ford stud wheels and used car tyres on it that were 2yrs old which needed replacing for rego). The result was the trailer sat 2 inches higher because of the light truck tyres. To combat the angle of the trailer I bought an adjustable ball mount from hayman reese for about $150 (#21228) and adjusted it to full height and now it sits level again.
Russman: My '09 VE SSV Ute sits similar to yours (not as bad, but i've had mine lowered with stiffer rear springs too) and thats with just a 550kg car on a 550kg trailer (sitting level). I'm gonna look at getting some different rear shocks or some other setup in the rear. It does tow perfectly though, can safetly do the speed limit and keep up with regular cars (which shocks them because they all pull out to overtake only to find they cant even keep up) and my fuel economy only jumps about 2.5L/100km over regular city driving (6L engine drops to v4 for quite a bit of the trip back to sydney from wakefield which helps the economy).
Still got 4yrs to go on my utes lease so after that I may look at a new Navara as well.
As for using a van to tow, no problems but you really need to put some weight in the back of them when you tow (spare wheels and tools would be ideal) otherwise they feel like the trailer is doing all the steering. They lack power but make up for it in reliability. (I traded my previous tow vehicle, a 2000 hiace with 370,000km on it for my ssv ute - the hiace was great, it just didnt like going up hills with a trailer on it haha)