Personally I'd tee it downstream of the changeover regulator, open both tanks & you should be good, if one bottle empties it automatically switches to the other. With 2 30lb (14 gallons) bottles you should be good overnight, or a few nights, even running the furnace, then refill as necessary when the indicator shows red on the regulator or upgrade to 2 40lbers (18 gallons) should get a week +. At night running the furnace, biggest gas hog onboard, shouldn't use much propane running the generator to keep batteries charged & every little other load.
Danny & Linda
Former '13 FB owner