It's been a while since I used the RTA feature, but there is an icon or something there that will freeze the reading. You can also save those readings to a jpg.
There might be an easier way, though, using just an SPL meter. Keep in mind that bass "chases" boundaries (sorry for the poor terminology). With broadband pink noise, and the meter set for C weighting, you'll find that the SPL reading will increase the closer you move to a room boundary, and more so at corners. As a "for instance," if you have a symmetrical room with shoebox dimensions, the lowest reading will be in the dead center of the room. Walk from there towards a wall and you'll see the reading increase. Walk from the wall towards a corner and the meter reading will increase further.
It's a little trickier in a non-symmetrical room, but the same roughly applies: the closer to a boundary you move, the more perceived bass levels increase.
Hope this helps.
Regards,
Wayne