I've never done this myself , but I'll tell you what I've seen.
Tire covers, some use fabric/vinal others 1/2" plywood cut to cover.
Reflective insulation ( hot water tank type available in 3' wide lengths @ Ace and other hardware stores ) inside all the windows.
Foil tape ( lasts longer than duct tape and easier clean up 3" wide roll ) covering all the furnace and H/W openings.
7 gal water jug ( full )standing in shower stall or tub with spigot removed, allows slight amount of humidity into the RV.
Bay leaves , spread every where on the floor, particularly in front of entry door.
Lube toilet seal to prevent it welding to the gate valve in the heat.
Close off all sink drains, to prevent " P " traps from drying out and allowing tank odours into RV.
Others will add more I'm sure, but this is what most of the snowbirds who store their units at the park where I spend the winter do.
Summer temps there hit 125f last year.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.