You shojld pull down the walls and insulate it if you are going to foam. You should also add windows. Picture windows are nice but you you can get windows that fit between the wall ribs that can be installed without welding.
Barn doors are easy for loading skis but there are definite advantages to a ramp. You save the space of carrying ramps.
But the biggest advantage to the tail gate is that you can add jacks and turn it into a deck. Many full sized toy haulers work like this. You can make a wall out of vinyl to hold the ac into the trailer, and also add an awning. It is a great place to turn wr3nchs and not contaminate your sleeping area. You can also consider building barn doors inside that you can close up the back to leave the tail gate down, turning the tailgate deck into a permanent screened room while camping. Add tie down into the floor of the ramp you chain down your toys while you sleep with a motion light.
I have barn doors, so loading the ski is easy, but i wish i had a ramp because i could add 64 sqft to my 144 sqft trailer.