I think all their current models are aluminum framed (the superstructure frame, not the chassis frame). Some older models (150,171,173) were wood framed.
Note that the 199FKS drains both sinks into the black water tank (which is 17 gallons), the shower is the only thing which drains to the gray water tank (32 gallons). Seems designed for people who mostly wash their hands in the shower. ;)
I've got a 177SE, the construction itself seems to be above average in my experience, but that's not saying much when talking about RV construction. The only issue I've had has been water getting into a couple of the exterior lights (but not past them inside the skin, AFAICT).
There is a
Riverside specific forum, people generally seem happy with them.