Ah, yes, been there--done that. So, chassis A/C is broken and going to cost a bunch to fix, eh? Trust me, just fix it. Now, or budget for it. Mine has gone down a couple of times, and I've tried the ol' run the genny and blast the rooftop. It just doesn't work that well. My guess is that it has to do with the total air volume inside of the whole motorhome that it needs to cool + air exchange and venting as that total air volume gets slowly vented out while heat from the sun beating down on the windows, skylights, vents, etc. + the extra heat from the engine/transmission/generator working while driving down the road. It's just a losing battle. It works fine when parked, so I don't think there is anything "wrong". Make sure the usual maintenance items are done...cleaning the coils, filters, etc. Here is a great video to help show you how to do that and to understand the system in general...
https://youtu.be/Gbm8hOyZlrU
Nothing is going to feel as good as having that chassis A/C blowing on you when driving though. It's either that or travel during cooler temps, like at night. Best-