We have a self build crafter LWB, converted in 2019; started with 600W solar 2KW inverter, 400Ah LFP. Gas underslung 20L, propex 2.8KW gas heater, propex gas/electric boiler. Two burner gas hob, Dualit electric oven.
After few years of use, we made changes along the way, longest in the van 6 months ( covid), but majority of trips up to 3 months now. Lowest temperatures we been in -22C and hottest 42C. Maxxfan over the bed made a big diference.
Tu sum it up in regards of LPG, now, we use 15-20L per year in colder months only, that is if we don't have enough electricity.
We added/changed to 24v habitation, 3KW inverter and 500Ah LFP at 24V. This, with 50A B2B, 600W solar enables us EHU free 99% of the time, and heating is done by diesel heater now. Hot water predominantly electric, compressor fridge/freezer, cooking 99% electric, gas is there as a back up now.
When this tank comes to end of life, 10 years, that would be in 3 years time, is coming out.
With price of Lithium coming down, we shifted most things to electric, and use very little gas, can do without it if need it to.
Heating, nothing beats diesel, even if is of the main tank, you wont make a big dent in the reserve. Longest we had the heater on 24/7 for 3 weeks during winter, on low. At a rate of 0.125mL per hr, that is 3L per day running non stop. Fuel stations are everywhere, not so much with LPG.
If I would build a van again in today's climate, it would be 24v at minimum or even 48v habitation, diesel heating and hot water, also electric, and zero LPG. The weight of a underslung can go towards extra fresh water.