I would seek proper advice as this could get expensive very quickly:
1. You've stated there is a covenant in place - the exact wording needs examining, does it say no motorhomes / camper vans or just no vans or commercial vehicles - then the argument is whether the Cali is a vehicle of the type thats not allowed.
2. The landowner appears to be the one making the complaint & presumably they are the ones who put the covenant in the deeds in the first place, its up to them whether they want the expense of enforcing it or not. This is not normally a problem as once the developer sells all the houses on an estate & moves on to the next site they couldn't care less about enforcing covenants, but in this case it looks like they are there to stay.
3. Their Landrover does not come into it, as they presumably wrote the covenants how do you know that their deeds have the same covenants as yours?
4. If there is an adopted road without parking restrictions outside your house, park it there, or better still park it right outside their house - they might then prefer it if you parked it on your own property.
5. Why not just ignore them? they will need to spend a fair bit of money to get this to court & as far as I know their costs are not recoverable against you, if you loose you've lost your parking space, nothing else. If you have legal expenses cover on an insurance policy you may well be able to defend an action against you at the insurers expense.