Fan works, but if A/C turned on, trips breaker within a few minut

You need to verify if the problem is the unit or the breaker.  To do this, you will need to measure the current it is pulling from the power source.
If the unit is pulling too much current, then the unit is the problem.  If the unit is pulling as much or less than its rated current draw, then the circuit breaker is worn out.
The units WILL pull more power when hot outside.  It should not trip a breaker unless something is wrong with either the unit or the breaker.
A current draw test will tell you for sure.
If it's not the breaker, then it's probably low refigerant causing the issue.

Check to see if the condenser coil is plugged it can cause your problem. Hi head pressure increases compressor amp draw there is no plug on a rv a/c.

I would replace the breaker s they are cheep do that first make sure the breaker is tight and replace the wall receptacle. The wall receptacle can be worn out from heating up and cooling and can cause the plug in to become loose and cause high amperage. Due that you are running it when it's hot the compressor would run more often causing the plug and or wires to expand and cause Resistance and thus kicking the breaker. At night when it's cool the compressor does not to run as much so it is mainly the fan running, and normally low refrigerant would not cause it to kick the breaker. Low oil level could cause this but not likely low refrigerant, also make sure the condenser is not plugged up with leaves or other debree's . Do this It is the cheepest to deal with a wall plug is $2.00 and breaker 20 amp is $5.00 a service call will cost you far more so if you can do those yourself you can save that much money yourself.