This is a longshot... double check the trailer and make sure the tongue is not a 2 5/16" hitch. I dont think popups would use that size but maybe if it is a big popup. Hitch couplers vary in design, some have an adjustment nut underneath that needs to be tightened to where the ball is snug in the coupler. If the design allows put a snap or a lock on the lever after it is attached, to keep the lever from opening.
Robert & Nancy with "Murphy the EOG"
Murphy has passed on, but Micah and Bogie have assumed the watch! 02 Holiday Rambler 5ver, 2015 Indian Chief Vintage. 98 Coachmen truck camper.