You're right - typically the generator in a Roadtrek is installed where the under-mount spare tire is located. The spare tire is relocated to a carrier on the rear of the van. So, if you don't have an above the frame compartment that is big enough for an Onan then it was probably an under-mount generator.
Hopefully someone who knows for sure will answer here.
You can ask on the Roadtrek Group on Yahoo also - http://autos.groups.yahoo.com/group/roadtrek/
Also, take a look at the 1991, 1992 & 1993 brochures to confirm your Roadtrek model. http://www.roadtrek.com/brochure_list.aspx
I see a 210-Independent listed for 1992.
My guess re: "generator ready" would mean there should be some unused wiring for the generator in the spare tire area under the van.