The only "less expensive" 395/85R20s in North america are Michelin XMLs - military take-offs. XZLs are hard to find, only Michelin dealers have new ones at the moment; for ~$1800 each.
You are not going to find 365s in North America, i think.
Vrakking/Aircrafttyres in the Netherlands, and other suppliers have XZLs in both sizes, new for ~900 euros, but the shipping....
When I had my U500 in australia for 2 years I had one almost new spare and 2 almost new unmounted spares on the roof of the camper. I did indeed switch them out on one axle, since I drove ~25000 miles on the trip.
Unimog U500 with Unicat camper; diesel BMW X5 35d, diesel BJ40 Landcruiser and diesel M37