You do know that whatever level of load center you have in your RV, you'll have to have the power source to supply it? IOW, your Onan may have supplied 50 amps of current but if you've downsized to a Honda 2000, it won't meet the demand.
To upgrade, you'll need to change out the load center, the power inlet and wiring, and buy a 50 amp shore cord. As long as you have everything in long enough wire lengths from each circuit to reach the breakers on the new load center, things could be easy. If you need to lengthen wire runs, then you will need to do some box installation and splicing. The nice thing about going to a 50 amp load center is that you can split off some circuits to another breaker to be better balanced.
An alternative, especially if the upgrade is only to solve a single issue like a second AC unit, would be to just establish another independent 20 amp circuit that is connected to an unused pedestal receptacle. How that would fit in with your generator is unknown 'cuz we don't know what you have.