Roof top AC

In a ducted unit it is important to make sure that the unit is sealed to your duct, with no leaks. This can be seen by taking off the inside cover and looking for loose tape, or other means used to insure that the cold air from the AC unit is going directly in to the duct and not leaking outside or into the roof frame.

At PPL in Houston the same AC costs $425 plus installation or shipping.

It is a ducted unit. I've cleaned the inside filter but haven't gotten up and removed shroud. What would I be cleaning? Is it a matter of blowing out the fins?

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.

have you clean the unit. Requires to get on top to do. Is it a ducted or open system. Checked filters. Yes you can recharge but the cost will be high. Even to check the charge you have to install a port. The rest is easy. Finding someone who will work on it could be an issue. True it is easy to replace.