In my opinion these mods are must haves for pop up campers.
A BAL leveler is a must for any single axle trailer, it makes leveling so much easier .
A digital thermostat will keep your trailer at a more constant temperature eliminating the cold time before the heater comes on. And get it up off the floor mount it on the sink cabinet.
Reflectix window inserts. I guarantee these inserts in your windows will help hold your heat in.
Popup Gizmos (pugs) or emergency space blankets over the bunk ends.
A door alarm, lets face it the lock on a popup trailer door is pretty useless, a door alarm will wake you up if your door is pried open.
And some 12 volt plug in so you can power some things off 12 volt power.