Need help, upgrading Dometic Analog Thermostat to Digital

What Old-Biscuit and Chris said should work for you. The main problem you're having is using the +7.5 rather than ground as the common. It's a very understandable blunder as the Dometic system is somewhat unusual (certainly compared to normal domestic thermostats) in switching to ground/common.

This thread describes what I did to update my thermostat; it's the same thing, but a couple of wire colors were a bit different.