If the unit's paper work says use a 27, I'd be after the dealer to rectify the problem. If an issue were to arise later where you'd have to make a warranty claim, there might be a problem if someone were to find out you only have a group 24. Personally I feel that type of attitude is BS, but that's exactly how some factory warranty repair approval people think. I used to work for a RV dealer so I have a certain amount of experience here.
As for reality, if you always go 'plug-in' camping, a group 24 battery should be fine. But if you ever go camping were no hook-ups are available, you'll need additional battery capacity. Without that 12 volt electrical source nothing will work in a travel trailer or 5th wheel. No refrigerator (even on LP gas), furnance, water pump or lights. Nothing, nada, zip.
Now here's one other situation to consider. Last week we were camping with an electrical hookup. During the morning I notced the refrigerator was running on gas. It had auto switched over due to a loss of power in the camp ground. It turned out power was restored in about 3 or 4 hours. But many campgrounds we all enjoy are VERY rural. It is not uncommon for extremely rural areas to lose power after storms in the summer. Depending on the cause, power may be restored in minutes, sometimes in hours or even days. Just sayin'.
FAA A&P mechanic, ex-RV dealer parts dept
Retired B-52 Gunner/Flight Examiner, B-52 Crew Chief
2016 Jayco Eagle HT 29.5 FBDS w/ 15,000 BTU AC, 50 amp service, upgraded counter top, thermal pane windows, elec. stab jacks, outside grill, backup camera and MORryde pin box.
2011 GMC 2500HD DURAMAX/ ALLISON