16" tires are much cheaper than 17". I have 17". I'm glad I did because by rear axle will top 10,000 pounds, but I pay every other fall when I have to get a new set.
For a coach of that year, even 16" tires will have much more capacity than the axle is rated for, so you won't really be able to take advantage of the higher tire capacity anyways.
2000 Ford E450 V10 VAN! 450,000+ miles
2014 ORV really big trailer
2015 Ford Focus ST