I have a screamineagle road king and there is very little room to get any pressure to pop them on.
I took a piece of flat hardwood put a little double stick tape in the end, put the cap in place then wedged another piece between the top of the frame and the one holding the cap. Takes a little finesse but it works. I have done this on other parts of the bike as well.
Just be careful to protect the underside of the tank.