The adaptors ; IF available; would move the tire/wheel out possibly as much as 2" and could make for clearance issues , on fenders , any difference in tire height could give clearance issues with your floor .
Difference in height , also effects , your differential gearing and aside from speedo problems , performance on hills and fuel mileage .
Any adaptor you consider , make sure it's rated for your RV weight.
JMHO: Keep shopping for 17.5" tires that suit your budget.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.