Cheapest place I've found for tyres is buying online from tyre-shopper.co.uk price includes fitting/balance & tyres are then delivered to nearest National Tyres depot to you for fitting. Obviously with any tyre depot you need to check if they can take a caravan but plenty can, or if you have the gear & space, jack up caravan leave on axle stands & you can take wheels in boot of your car to tyre depot.
Or you can use a mobile service to come to you like e-tyres but I've never found them the cheapest. Whatever you do ensure you get them balanced & with new valve. A tyre depot will do this as a matter of course. If you have them changed at your caravan service place as part of a service they may not get balanced. If you are going to do plenty of motorway mileage I would get at least a mid range brand fitted.
I have certainly learnt a lot from this thread, the first thing I'm going to do when I get the Caravan out of storage on Wednesday is inspect the tyres not just for damage but there date too....
We saw an example of this last night on our way back from an overnight stay at Blackpool.
We were very near to the Knutsford south bound services & we had to slow very quickly as the caravan which was in front of us & a few other cars by approx 150 yards had this very thing happen to them!
The drivers side tyre was in shreds over the road & also some of the caravan body / wheel arch.
I hope all involved were ok.
Keep safe everyone.