Turns out I had the stuff. Some sort of metallic tape...
Here's the taped up ducts:
And just to help out with the return, I blocked off the holed made for the wiring and sealed the gaps. This should help the condenser stay clean, I don't want to clean that one.
Not pictured is the divider. When putting everything back you need to be sure that the divider is sealed properly. If not, cold air can enter the return and freeze the condenser.
I tested the fan operation and it seems to blow a little stronger, like mentioned before, RV A/C's are not the most efficient, so EVERY little bit helps, and it's quick and inexpensive (since I already had the tape) and it just makes sense (to me anyway).
NOW my A/C's are ready for the summer.