Rather than a grinder, think about using a wire wheel; that way you are not taking any more metal away (safety glasses, face shield and dust mask are not optional!). If your time is more valuable and you are pulling the cabin apart/off, you may want to consider going to a bare frame and having it blasted. It would be more effective at eradicating the rust and preparing the surface for a good bond with your paint of choice. A lot easier to get to everything and work on it, too.
If the pitting is just light, does not thin the metal and does not create a flaw on an edge where a tear or crack can start, then it is probably okay; you will have to be the judge or get someone experienced to take a first hand look for you.
A bunch of deeper pits, thinning and/or flaws at the edges can weaken the metal a bunch.
The coupler area looks fine to me. The areas underneath aren't as good (duh, stating the obvious
), but I don't see anything too scary in the pics that you have shown.
If you are doing a "historical restoration" you can probably save it all, but if you are building an heirloom camper for reliability to have fun in for many years, I would take a serious look at everything and consider replacing any suspect pieces. If you find any major issues... well, we can address those when they show up.
In the end it will probably be fine, and I have not helped you in the least at making a decision, have I? $.02
