Dedicated 20AMP outlet from Generator

You probably aren't going to find anything significantly smaller than what's available at local big box or electrical supply places. Code specifies so much space for wiring around breakers, so there's not much of a market for smaller (non-compliant) sub panels. That said, the two or four breaker boxes don't seem overly huge to me.

For an outlet mounted on the exterior of the RV, don't forget that it ought to be either a GFCI outlet or equivalently have a GFCI breaker. IMHO it's not a bad idea regardless of where the outlet is located.