For me ,it depended on where I was going to use my trailer the most.I boondoggle about 90% of time ,so I went with 12vdc heaters.The hardest part was removing the underbelly cover to get to everything.My heater duct work goes under also, so that helps.Amp draw depended on length,if I remember correctly,nothing major as long as you do not install too many.I have 4 Costco batts That are 5 years old and they can still handle about 2 nights worth.I turn my heaters on before it gets cold, esp if headed to Flag,Turn on at house and when it starts getting cold enough for them,they come on.I think about 40 degrees.Besides, when pulling,truck handles amp draw.If I was going to camp where power was avaialble I would go 120v.Things I learned,start heaters early,belly has to be covered,if possibly,keep wind from blowing under trailer(learned that at Alamo Lake,Great v iew,but wind whipping under trailer made it impossible to keep temps up)so camper location from wind, and do not let temp in trailer get too low.If I was doing again I would take more time and do better wraps,also take pictures where everything is located and connected once finished.
