The 14x14 opening is standard for all RV AC units. Your choice includes BTU size and either cool only or cool&heat (heat pump) functionality. There is a choice of manufacturer as well.
Personally, I am of the opinion you should move up to the 15k BTU size for a 31' trailer. I have had both Dometic and Coleman HVAC dual temperature (heat pumps) models and found both satisfactory. Availability and price point would be the key decider for me as inventories and the supply chain are in disarray.
"Good judgement comes from experience. Experience comes from bad judgement."
Air #48582, S/SO #003, WBCCI #4584