If the A/C is in the very back I don't see an other way other than getting a second A/C, Even with all other vents closed, air will still leak out and it will warm up as it passes through the ducting. You could get a 9k BTU and use that just for the Bed Room. I had a similar problem with my 5th wheel. Even though It had 2 A/C's they where both in the front most half of the RV and too cool the bunk house I had to run both A/C to get the air to push out COLD out of the 2 bunk house ducts. But then I noticed the pre-wiring for a 3rd and got a 13.5k BTU Power Saver model which only pulls 11 amps vs. atleast 15 amps for normal 13.5k and 15k. And I plugged up the ducts so each A/C has Its own zone. So now the cooling is more effective and controlled.
2013 OpenRange Roamer 395bhs
2022 F350srw Limited, tower
2021 F150 Platinum Hybrid, daily
2002 Accord V6/5spd, project
2014 Kawasaki Concours