Refrigerator Problem

I got confirmation from Dometic that this fridge should operate without problems up to 9000 ft elevation on propane. I checked the propane pressure in central TX (about 800 ft elevation)and it gave 11-11.5 inches of water which is within specifications. So it appears that my propane regulator may not be putting out the correct pressure at elevations above 6000 ft. Existing regulator is a Marshall Gas Control Model 299 which is very cheap.

Any suggestions on a higher quality gas regulator that might work better at higher altitudes?