I bought the JT Strong arms for my 07 Montana 3585SA. I didn't have any problem installing them on the front frame but I can't pull off the underbelly in the back to find out where the cross member is. Does anyone have any suggestions.
When I installed mine several months ago, I was able to locate the cross member by pushing and tapping on the underbelly. I had the underbelly down today measuring for a bike carrier and noticed I was about 1/8" off from the center of the metal with the screws. There actually is not a true cross member in the rear. It is a metal floor joist with 1 1/2" wide angle iron on the bottom. The center of the angle iron is 3" from the edge of the rear cap.
I took a couple of pictures today that might help you, but forgot how to post them. Will try again to post them.
I got some tips from Mudchief - when you locate the crossmember, use a Knife to cut a "U-shape" slit in the underbelly in front of the crossmember, enough to get up there with a wrench and nut. Once installed, seal the slits closed. I used the black firewall sealant from NAPA.