If the ramp is very steep, use a bale of straw, or blocks or whatever appropriate, to level it a bit; pigs will happily climb up onto the ramp that way, its the steep bit they hate. Try taking them out before feeding, put a small amount on the ramp, once one is on put some further up, etc. etc. when they are all in, dont shut the ramp but feed them in the trailer. Repeat as often as possible. It works for us.
NB When we did not have a trailer available we used a van, put bales up as steps, pigs leaped in. Enjoyed the experience. Need to take bales with you for exiting though.