I wouldn’t try R plus if the owner wasn’t committed to it. It’s just going to cause problems and disappoint the horse. It’s also amazing how many horse owners don’t have the timing and intuition to shape a behavior.
As far as when you reward, you need a plan and you need to decide how to build up to a behavior. You might very well have a plan that has stages. Stand quietly at the bottom of the ramp. Put one foot on the ramp. Put two feet on the ramp. Stand on the ramp. Etc. Also you could incorporate following a target stick up the ramp if the horse understands that. Or standing on circus boxes away from the trailer.
Depending on the depth of the behavior you want to change, it might take many sessions to change how the horse responds to the trailer. And you can’t give up and revert to pressure without undoing your work. You need to change how the horse feels about the situation.
Not every horse is ready for clicker training, but absolutely done right it can stop them being mouthy and pushy