A/C off of 20 amp service?

It is much more likely the salesperson said to not run the AC on a 20 amp connection because there is very little margin for additional loads on 20 amps and it is highly likely the breakers are going to trip at some point. That isn't a problem in and of itself, but many appliances and the like have cool down periods and de-powering them in mid operation (which is what happens when a breaker trips) isn't good. Also, if breakers trip while the rig is unattended, there may be other things that can be harmed. The chassis batteries may very well totally discharge, again not good. If the fridge wasn't dual fuel, or didn't properly switch over, food could spoil. Pets could get overheated. If you think your rig is getting shore power and it isn't, there can be problems.
In my opinion, if you are operating the AC in an RV on a 20amp circuit, you need to monitor it frequently at the least. Not everyone would do that and that is where trouble would begin.