Yep there is a point of diminishing returns where a heat pump will no longer be able to extract enough heat from the outside air to give you adequate heat discharge.
The other thing is that those outside coils that usually are hot in the summer, exhausting the heat from inside the trailer are now cold instead. Depending upon the outdoor humidity the outside condenser can form ice. That's why most heat pumps go into a reverse mode after a certain amount of run time, to clear the ice off the outdoor coils . In extremely humid situations and colder out door temperatures, there also is the possibility of the defrost cycle not being long enough and your coils may ice up. We had this happen in Branson once on a cold rainy day. We came back to the trailer and found the trailer cold inside. I went outside and noticed the outdoor coils were completely white with ice.
'04 Classic 30' S.O.,'03 GMC Savana 2500,'14 Honda CTX 700