The Chinese night heaters are fine I've fitted a few now,if buying new try and get one that comes with a small (around 16 mm from memory) nut on the pick up pipe not the larger plate with 3 or 4 screws the nut type fits the boxer tank easily.Brave idea to try water heating as well ,I thought about that but decided too much hassle and not likely to be good enough..being tight I went with a carver( actually bought 3 in the end for circa £200 so plenty of spares although most are still available) 240v/ lpg combined with a Chinese digital temp controller (under £10 fleabay £5 temu) beware some lose settings if power removed,left normal carver system in placejust put digital in main feed to carver control.End result..cheap compact proven simple controllable within 1 degree.
I have Renogy b2b mppt...simple very well built and keeps all batteries topped up when van idle.renogy app is ok,but you may want to add a
battery meter (
shunt) plenty Chinese around £20/30 see utube..you will then know state of charge,also how about charging from hook up( buil in a ytandard smart charge ( mines 50amp .again £30/40 Ali/temu( lithium profile)I have 400w
solar pluss rigid folding 220w and l/a 220ah, but going to upgrade to 300ah lithium (£300) with BT monitoring.Go sureflo and pressure vessel and domestic fittings,longer term you will find air assist almost essential my van is 2020 24 K miles and sits low at the back but is not overweight air assist vast improvement only mentioned as you. May want to add pipe wire now in readiness( mine compressor under bonnet dual digital gauge and 2 (up/down l/R) switches on dash,control pack (4 s/v) under bonnet 2 air pressure sensors top of fuel tank.
I used 2 leather bmw seats all singing each has built in compressor £75 the pair modified drivers and double passenger seat bases from van( sold drivers original £300 ..a y one want half a passenger base
.
Good luck with your project!