The stuff to use is PU sealant. The price of that stuff is ridiculous & useless for any sort of caravan applications. £38.77 incl postage to cover less than 1 sq mtre? They are 'avin a laff.
You can do a whole caravan with a cartridge of PU sealant for about 7quid.
I've been buying cartridges of PU18 sealant from a company in Redruth. Seems like a suitable sealant for Caravan use with good longevity and a much better price than some of the better known brands.
Been using it for the last month during the front end fix-up of our Abi caravan.