They are correct when they say it's a closed system, however some have had fittings brazed into the lines so that recharging is possible. This doesn't fix the leak but does allow for recharging.
I've read where some posters have been disappointed with the cooling power of newer AC units. I have no 1st hand knowledge but it might be worth some research by you to see if it's one AC manufacturer in question or all of them.
I bought a used Coleman off EBay a few years ago and it was running strong when I sold my TT. Just another option.