looking to change our caravelle for a california (felt needed to justify why I'm on here) our velle has 20 inch wheels but on our ski trip we put winters on it. This is for 3 reasons
1 decent chains are really expensive
2 never checked but not sure I'd get chains on and be able to use full lock
3 based on point 1 a set of winter tyres and wheels were similar money to quality 20inc chains.
Having driven with both on the winters are brilliant and because they're narrower exert more pressure. Whatever you do buy make sure they are suitable for commercial I think 16mm not 12mm as having seen both mine & others destroy cheap chains leaving you potentially stuck. Treat them as an insurance policy ie buy the best you can and if you don't need them resell. There are a coup,e of companies who rent them good luck and have a great trip.
Don't forget your 20inch tyres will also do 4 months less mileage but your van won't look as nice !!