Firstly, don't level the caravan via the corner steadies at all as the levellers are only bolted to the wooden floor and not the chassis. You either need a proper designed plastic ramp or blocks of wood under the wheel that needs raising which you will need a spirit level to do this. You pull the caravan up the one ramp with the towing vehicle till the caravan is level, apply the caravan handbrake, unhitch from the vehicle and then level the caravan from front to back via the jockey wheel. The caravan will then be level both ways.
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You'll need to level your caravan side to side first i.e. not front to back. If you have a mini spirit level set in the front window this will make it easier. You'll need wooden planks or plastic levellers under the wheel (or wheels if twin axle) on the side you need to 'bring up'. The corner steadies are just to steady the caravan so should not be raised too high and should in no way used to take too much strain. It might be best to put wooden blocks under each steady as I was advised not to wind the steady down so it's nearly upright. Once the caravan is level side to side then use the jockey wheel to level front to back. Once you've done this then lower the steadies. Don't forget to put the handbrake on as well. Good luck.
If you are setting caravan up for a long period in your garden then jack it up with trolly jack if you have one and put concrete slab or wood plank under wheel on one side. Experiment with thickness to get it level side to side. If you have no jack then tow caravan onto slab with car. Use a spirit level to check.
Another method if you are not bothered about grass is simply to dig a hole and tow caravan forward and drop wheel into hole. Mess about with depth of hole to get it right depth for level caravan.
Once you are level side to side, adjust level end to end. Use that vertical winder thing on front with small wheel on. Thats the jockey wheel.
So now you should have a totally level caravan, yes? Look under each corner of caravan, see those screw thread legs? They are the corner steadies. Put wood plank or paving slab under each one and wind down each leg. See that nut on end of each steady? You should have a winding handle in caravan. Use that to wind legs firmly down onto slabs.
Check your levels again with spirit level and tweak steadies for final perfect level. I don't think caravan casette toilet can be connected to main sewer.
If setting up for a long time it is better to lift it up onto axle stands to take the weight off the tyres or else they will be ruined if sitting on one place for ages and ages.