Hello Matthew,
Reading of your situation really concerns me. Manhandling even a small caravan on a 1in 5 is highly dangerous - consider if one of you looses their footing, the weight of the caravan would all too easily over power the other person, and I dread to think of the possible consequences of injury or even damage.
This no only leaves mechanical assistance if you intend to persevere with you own storage.
For space reasons you have discounted using a vehicle, so that leaves winches or caravan movers.
A winch basically only allows you to pull in one direction so if you need to turn the caravan to park it then your back to the manual handling concerns.
This now leaves motor movers. There are basically two types, those that fit to the nose of the caravan which I generically call A frame movers. And those that drive that drive the main wheels that I call Axle movers.
Whilst the ability to start to move a caravan is of course important, of greater importance is the ability to stop or control the caravan once it is moving. Even a 800Kg caravan on a 1 in 5 packs a lot of kinetic energy which has to be controlled safely.
Most movers have plenty of power, but power is no use if you don't have the traction to be able to use the power. In fact without going through a lot of physics, the maximum traction or Drawbar force can only be great as the down force on the driven wheel. Not enough down force and the wheel spins. The same applies to braking, so stopping the movement or holding it on a slope is also related to the down force.
Now consider an A frame mover - if the nose weight of your caravan is say 75Kg, and the weight of the mover is 25kg that is only 100Kg on the driven wheel.
On a 1in 5 slope roughly 20% of the down force is lost because of the resolution of forces in a triangle and equally 20% 0f the caravans weight is transferred as force down the plane of slope, so that is 160kg trying to push the caravan down the slope with only 80 to 100Kg load on the mover wheel, there is a very real danger of uncontrolled skidding. Add to that the additional loss of traction due to wet surfaces or leaves etc and in your situation an A frame mover looks like a non starter.
By comparison, an axle mover has roughly half the total weight of the caravan on each wheel, so there is a very significant safety margin against skidding.
The whole principal behind the original Carver caravan mover was to enable the caravan to be moved up a 1 in 4 slope, a feature that has been covered by most movers, and that was with a bigger caravan than yours, so an axle mover should be fine for your needs.
Please don't go away with the thought that I am against the A frame movers. They definitely have their place and uses, but its horses for courses, and an A frame unit I believe could be a recipe for disaster in your given circumstances.