I don't have the same model, but my 3818, has 85's on it with no problems! The 85 shows .9" taller than the & no wider so I'd say it will work fine! My trailer has Goodyear G614's on it & so far they seem to be a nice tire!
I just put 6 new Sailun 235/85R16's on my 3990 no problem, plenty of room. It had 235/80R16's by the way.
Chris & Donna
2014 Voltage Epic 3990, 600AH Lifeblue, 1895W Solar, 3K Magnum w/AGS, 3X Micro-Air ES
2018 RAM 3500 Laramie Sport Crew Cab 6.7 Cummins, AISIN 4X4 DRW 4.10 Long Bed Dually 2004 Sportster 1200c
I've been running a Chinese tire by the name of Pro-Meter, 14 ply, all steel cords, cost me 200$ each installed, 4 years old, 25000 miles and around 70% thread left on a 3905 Voltage toy hauler. These tires are a serious chunk of rubber, after 4 year the still look great, not like the 14 ply Good Year G159's that I had that crackedup in 4 years and started to decap....