not owning an aqua hot system, but understanding that your holding tanks are already within your basement environment - either heating the basement itself, or using the electric element for the tanks themselves, will keep the tanks above any freezing concern.
I would suspect that if you are plugged into shore power, the electric tank heaters would be the way to go - if temps are REALLY going to be that terribly low for a terribly long time.
If you are not on-grid, then using the aqua hot burner to keep the whole basement warm would suffice.
While our Palazzo uses more of a 'passive' tank warming system, which is basically a furnace vent to the basement from the propane furnace, but ONLY when the furnace is being used. There is no separate 'heating' element or tank pads for the holding tanks.
What I would admit, though, is that it's very highly unlikely that either holding tanks will be in any rare situation when the outside temps are so low, for so long, that the interior of the basement would drop to such low temperatures that the holding tanks would come anywhere close to 'freezing'. Most of us who've had issue with 'freezing' while camping or full-timing is typically only the outside water hose we either forgot to remove for the night, or forgot to insulate.
We've traveled in ALL kinds of whether, and have full-timed, like right now, when overnight temps fell well below freezing for many, many hours, if not all day long, and have never had any issues with our fresh water tank, or holding tanks. Interior tanks rarely have any issues with low outside temps, unless you've left an outside door open.
'14 Palazzo 33.3 bunkhouse 34' diesel
KingTailgater2 Dish HD, 100w/5a SOLAR, BlueOx
100,000+mi since '14 - US, Alaska, Canada - Northwest, GA! (also, '14 Gulfstream Amerilite and '07 ForestRiver Rockwood) : )