JDL (gotta love those initials) -- I measured the battery box opening on my 2007 20' to be 14 3/4 x 13 inches. There's actually a bit more room inside, as the hinged lid encroaches on the 13 inch dimension, so it might be possible to install batteries slightly longer that 13 inches.
At any rate, a pair of Interstate SRM-24s could be replaced with SRM-27Bs at 12-3/4 x 6-3/4 x 9-1/2 each. I don't know if the SRM-29 would fit at 13 x 6-3/4 x 10 because they might prove to be too tall (I neglected to measure the available height). It'd might be pretty cool to have the 29s, just looking at their 210 RC rating, compared to 140 for the SRM-24
For sure a pair of interstate SC31DM (or DA) AGMs would fit at 12-13/16 x 6-1/2 x 9-3/8 each, but I'm not prepared to give an opinion on AGM v traditional deep cycle batteries...
Of course, all this ignores the post positions, as noted by others. Have you made your choice by now, and what did you end up with?