As mentioned, it's no problem going from 50 to 15 (you should always carry those adapters anyways in case you aren't able to get 50A (some parks / sites are only 30A or sometimes there's problems at the post and 15A is the only one that works).
As far as not throwing the breaker, you should be fine but depending on your equipment you might need set it so that it doesn't try to use too much power for charging (our inverter has a 140A charger and I suspect that it will use more than 15A incoming 120V alone but it does have a setting (through the panel) to set the incoming voltage to 30A or more, 20A, ..., 5A