Heat pads......some water in the tanks...all of them
Electric supplemental heaters...great provided the RV furnace is set lower so that when temps drop the RV furnace comes on (it is the primary heat source to keep plumbing lines from freezing)
'Fan ON'......that will turn on the A/C unit blower.......which can help circulate the warm air thru-out RV even though the ducts are in the ceiling. The A/C return air is also in the ceiling and warm air rises so it will circulate it around the inside of RV.
Furnace fan only runs when furnace is operating so it is the only source to blow warm air into enclosed underbelly.
With the 'Yeti' package you should only need to blow warm air down there occasionally. Are you going to be connected to shore power? Don't know which ones you have..120V AC or 12V DC heat pads etc.