welcome to the site
I know they make and sell 5 th wheel tailgates for all sorts of pick up trucks that will be a lot less tall as a OEM tailgate, not sure if this would help you here or not
is your isue now that when tailgate is down it hits your front of your trailer??
as if so, pending the weight of your trailer, they do sell ball holder extensions, that can add some space between truck and trailer
but might cause some issues with tongue weight issues??
all depends on how heavy your trailer is
NEXT, IF your ATV's rack is just hitting the top of your tailgate and not allowing you to close it when atv is in, why not just add something under the rear of the atv's tires to get rack up above the tailgate??
maybe some cut up 6x6 timber or old railroad ties?
you can also, maybe get a sheet of treated ply wood, cut to desired length, add some 2x10 the length needed and have that as the floor of your trucks bed, using the 2x10 to hold up rear tires screwed to the ply wood(like 3/4 inch treated)
then just take your OEM tailgate off all together's long as you strap atv to bed good with some pulling going towards cab, things should stay planted real well
just have to be careful when loading it all doesn't want to tilt while driving up on ?
they also sell ATV beds, that mount over the top of your trucks bed and you drive atv up on from the SIDE of the truck
most are NOT cheap but??
or can load from rear too and be two atv wide deck?
I would think these will make for some top heaving hauling and might need some air bags to help control sway, but many guys like them for snowmobiles I know that
I knew a few guys in Canada back in the late 90's that had these for atv's and still using them now?
but I did see some VERY close calls flipping em over when driving up if not paying attention LOL