Sorry to hear that you had the damage! Did you have the hitch slid back as far as it will go before you started backing up the incline? I would think that would give you more clearance over the tailgate.
Do you have the necessary clearance over the bed rails when sitting on a level surface? If not, then you should probably raise the hitch or lower the pin if possible.
I do not have a 5th wheel trailer but have towed several in the past so my knowledge is limited.
Other than that, I can think nothing else to do except repair the tailgate or get a new one. Sorry!
Barney
