Our '06 Monterey is 109 gallons. I think that is the same across all submodels, and likely the same as your '05, as Stan comments.
Like Tom, except in rare circumstances, I start looking for fueling locations when the guage is at 1/2. I can usually count on 50-60 gallons going in at that point. As others point out, there are guage inaccuracies; other than the guage itself, these could possibly be due to long double filler neck tubes, the effects of foam-back and temperature considerations at fillup, or other variables. Your guage probably is just as good as any of ours, Dennis.
Joel