Please understand, I'm into Class C's and most of those are Ford E350 and E450 "van noses." For those, Ford wants at least 32% (beats me why they can't say "1/3") of loaded (truck scale) weight. You're at 31% which seems marginal at best. I'm not saying get to 33% and your worries are over. We tried to help somebody with a Class C that was in the high 20's %wise and never could get it to quit wandering. He even loaded 500# of pottery clay in the cab and it didn't help. Sold the coach. Instead of the 32% of total weight, some say the front axle should be loaded to 75% or more of its rating (GAWR). It likely works out about the same.
Based on what I've seen, done and read, I think:
1. Front axle is light
2. Rear overhang is too long or loaded too heavily
3. Rear springs and/or attaching components may be soft/loose
4. Narrow track contributes to the problem.
I still say call Henderson, Eric Davis or another specialist and be ready with a list of what you've done, the weights, your wheelbase and axle ratings.