We have camped in our MAXX when the weather was in the "teens". We used the builtin fireplace to supplement the furnace and stayed warm EXCEPT, like every RV we've owned, the floors get cold. The tanks did just fine although we did have the water line freeze up ( didn't break) close to the low point water drains. Also, you'll want/need a heated water hose or disconnect from water source. Our water pump is in the unheated storage compartment below the bed so we choose not to use it. We purchased a heat tape and insulated both low point drains and this fixed the freezing issue I mentioned.
Any RV will be iffy when the temps reach mid to low 20's or lower so you'll have to simply give it a try and add heat strips to any area that freezes. IMHO, it's a pain in the butte to camp in freezing weather and we try to avoid it on our winter trips south but do get caught now and then when we camp in and around Dallas. We would never purposely camp in freezing weather if it can be avoided!!!!!
Days camping (2016)----181 days
Days camping (2017) --- 82 days
2016 Wildcat MAXX 28RKX (33' TT), 2007 13' Scamp
2015 Ram Laramie Hemi, w/air suspension
30 years RV'ing
11 different RV's