Level indicator
Yes, I have heard of all of the alternative methods including using a two way level and using a Smart Phone, but my home-made device is certainly a much quicker and more accurate way, plus it works from outside the caravan in daylight or the dark - built in....
It is a basically very simple idea, just a tiny brass weight dangling inside a plastic tube, sat on an adjustable tripod mount. Get the weight to hang centre of the tube and if the tube is vertical to your caravan, then your caravan is as near level as you can get a caravan - except you cannot see the weight inside the tube....
To get around that, you suspend the brass weight via a very flexible bit of wire, from the top / middle of the round tube. So it can make contact with any of four contacts at the bottom of the tube. 12v positive to the top, then a 1k resistor in series with an LED (x5) which will light up when contact is made, with each, plus the fifth LED as a centre plus power on indicator.
The tube I used 8" of plastic conduit for, with a bush at the bottom, to mount it on a two alloy plate adjustable tripod. I drilled the bush 4 times, around the perimeter to take four screws the ends of which I filed to a point. That is the sensor part.
The screws are then finely adjusted so that the weight can only just not make contact with more than one opposing screw at a time, but with the very tiniest of gaps. The display LED's are mounted at the end of some very flexible 6 core wire, it shows the four LED's (for left, right, front, rear) plus an on/off switch. My sensor is screwed down in the front eye cupboard, with a plastic ice-cream tube upside down on top of it, to prevent it being disturbed, with a 12v supply going to it. The flex to the display, allows the display to be hung from a blind, in a front window.
The tripod plates are just two triangular pieces of alloy sheet, a spring, three nuts and bolt at each corner - adjusting the nuts against the springs, to fine tune its level.
First you set your caravan perfectly level, so it all drains nicely, then you adjust the tripod so the pairs of LED's flash with the slightest motion of the caravan. A solidly lit LED indicates it is high on that side, flashing alternate LED's then indicate perfectly level in that plain.
This gadget is now installed in my third caravan, as been it use for decades, making setting up on a site an absolute doddle.