Mixture of ATF and nail varnish remover, paint it on the threads and wherever you can get it, then bash the criap out of the end of the big bolt-head and around the sides of the nut, hitting the flats directly. The idea is to knock them just a tiny, tiny amount out of round to allow your easing fluid into the gap. Get two long breaker bars and extension tubes, if necessary. Don't worry about trying to use two bars at the outset, you need to get the whole shebang moving a bit. First, put a bar and extension on the outside hex head, extend to the max in a clockwise direction then stand on it. The idea of tightening it first is to break the crust of rust. If it cracks a bit and/or turns you are starting it off. If this fails, jack the back of the Landie up, rest the bar on the ground or the top of an axle stand, to your right, then let the Landie down using its weight to move the bolt and nut. Once it is on the move, rest the end of the back bar on the ground on the left, looking at it from the rear, so that as you move it the back one resists. Then stand on the outside bar in anti clockwise direction. They are really big nuts and bolts so I doubt you'll shear them off. If you do well, new ones can't be that expensive. I had the same problem ages ago and had to do this. A heat gun will not set your fuel on fire, I doubt, but there is only any point in using it on the nuts, you don't want to heat the bolt. But if you heat it up then spray the bolt with something cold, like WD or even iced water, it may help to crack the rust. Lock washers will give you the confidence of the assembly on reassembly but copperslip on the nut will help to get them undone again. I personally put two nuts on, as locknuts to ensure my FO big trailer doesn't go on holiday without us!
