Did you make note of which color wire went to which terminal on the old trailer thermostat before removing it? If you did, it should be easy to cross reference those connections to the new thermostat.
And did you put batteries in the new digital thermostat? It won't work without them.
And you are correct, the +7.5vdc line should be taped off and not used. Use a multimeter to verify which wire is the +7.5vdc line, but it is usually red.
Was the thick red wire connected to anything, and what is the voltage on it? Unless there is something else connected to your thermostat, I don't think that there is any need for +12vdc here at all.
On my '99 T-2453, this was the wiring:
Function Color Hunter Terminal
Hi Fan......BLU......Thru spst switch to G
May I also suggest that you ignore BLU (High Fan), and concentrate on getting the basic set up working first, and then add the High Fan capability after you know that everything else is OK.
'12 F250 4x4 Super Duty PowerStroke 6.7 diesel
2011 to present: '11 Cougar 326MKS
1999 to 2011: '99 Sunline T-2453
Amateur Radio kd2iat monitoring 146.52