I have recently removed the original Armstrong Rooftop AC unit from my 66 Tradewind and am cooling my trailer with only a 5k window unit. I started a thread if you are curious. I have tinted windows, keep the curtains closed and have LED lights. I will be replacing all 3 vents with Fantastic vents plus one more where the AC unit was located for a total of 4 FV. I will insulate these vents with an rv vent insulator I just ordered from Amazon. Here is the link:
Here is what I recommend for your trailer:
1. Tinted windows
2. White roof coating
3. Keep curtains closed on windows that you are not looking out of.
4. Insulate all windows that you are not looking out of with reflectix (bathroom, bedroom, etc.)
5. Insulate all vents.
6. Only use LED lights- They give off only about 10% of the heat compared to halegan and incandescent light bulbs.
7. Park in the shade whenever possible
8. Don't cool bathroom or bedroom during the day unless they are being used.
9. Use a fan to circulate the air in the trailer so it feels cooler.
The last change I would make would be the 15k AC unit. I don't think you will need it if you make the above changes.
Good luck, Dan