On the fifth wheel side there are lots of trailers that would push you into dually territory, but on the bumper pull side I don't think your truck will limit you at all. Especially since you're trying to stick to a shorter unit.
If you decide you want to supplement the rear springs a bit I really like the way timbrens work with the coils on my '15 Ram. I installed them without the spacer so the ride is unchanged empty but more settled at highway speeds when loaded with my fifth wheel. I got them instead of bags primarily because they're simpler and maintenance free.