Your answers will vary based on lifestyles, but for us the shortest we went between tanks while in the Alabama winter was about three weeks.
That said we supplimented our propane heat with electric heaters. We have two tower heaters with digital thermostats, one in the bedroom and one in the living area. They gave us an approximate warming of 20 degrees above the outside temp. We also have the electric fireplace which can give us a greater than 20 degree warming. However, the thermostat in it is not digital and is very hard to regulate.
Our practice was, during the day while we were out, the propane thermostat was set anywhere from 65 to 68 depending on the forecasted outside temp (we also have a dog with us). The fireplace was off and the living area tower heater was set to 68 and the bedroom tower heater was set to 66.
In the evening, while "at home" the fireplace was on with the propane set at 70 and the bedroom tower heater always at 66, unless it was shower time.
Just prior to shower time I would close the door to the bedroom and crank the tower heater in there up to 72 or so. Don's a man, tough, he didn't usually need the bedroom warmed up for his hower.
Almost forgot, we also have the heat pump on our upstairs A/C unit, but it's very loud and we never used it at night and only ocassionally while we were awake.
We have found no negatives with the insullation although our salesman recommended having an additional layer of home insullation put in the underbelly and we did so. We have never had to drop the underbelly, so can't say exactly what is down there. We also have 12v heat strips on our tanks, but have never had to use them and we've seen 17 degrees or so. We do have the thermopane windows and certainly beleive them to be of help based on our experience with our previous 5ver.
We do not have the W/D so we can't be of help there. We chose to go the laundromat route because of the extremely small size of the load capacity. Instead we put a small freezer in our W/D space and find the extra frozen food more beneficial.
We have met fulltimers in their early to mid 70's who are still going strong. But, we are late 50's, early 60's so hope to have a few good years left.
Just an opinion, unless the '06 Landmark is a Shenandoah, we would would upgrade to a 3500 dually. Again. not trying to be the weight police, just our preference.
Good luck on making you decisions and Happy Highways!