bjames, I believe that Surveyor has only ever used 6-gallon water heaters for all of their trailers. Your hybrid most certainly has a 6-gallon OEM from the factory unless it was replaced with an after-market 10-gallon.
Here's a brochure from 2005 in case you are interested.
DW, 3 Kids and our Goldens
2012 Shamrock 233S
2008 Toyota Sequoia 5.7L 4WD