Cost to add AC Unit

I added a 2nd A/C in the bedroom in 2016. Got it from e-bay. It was a dometic 13500 btu. It was not a low profile. Measure your rig, you probably do not need the low profile, and the cost is much more. I had a maxx air cover over the vent and it was about the same height as the new A/C. I paid around 500 for the unit & shipping. I just looked and they are around 600 to 650 now.
Check around your vent inside and see if there is a junction box with a plastic plate over it. That will be the box with the pre wiring for the A/C, and access is easy enough once you remove the existing vent. My 5er was a Keystone Everest 345 and about 40' long.
Its easy enough to do yourself, but will need help to get it on the roof. Use a ladder and rope and pull it up. The next hard thing is getting he old vent out. They use tons of "goop" or sealant to seal the existing vent. That takes a while to get it all off. Then put the new gasket around the vent & sit the new A/C on it an the rest is done inside. I'm no electrician or rv tech. You can do it, just follow he instructions.....

If you remove the interior bezel from the vent over the bed you will be able to see the wiring if it is pre wired. You can go from there.

Probably one of the best low profile units is the Penquin( id go with the 15000 btu unit)...nice looking but on the high end price wise.

It took me a couple of hours to replace my unit. RV Country is the go to, like Rob said.

I,d call a mobil rv tech ,get a quote they come to you. and I,d bet the wait would be much less than a dealer

You can find them under $600 by doing a quick google search from many online RV parts stores. RV Parts Country has a Coleman for $558 and another $99 for the ceiling assembly. Are the camping world units both the roof part and interior ceiling part? There's several Youtube videos showing how to install them. The hardest part is getting it to the roof. They aren't light!