Seriously Dazmond have you lost the plot ? an immersion heater will cost you a few hundred quid, do the same job, and you
wont have to drive your van until it falls apart, £4.5k for softer hoses and a couple of first cleans a year, (please take this constructively ) mate you need to give your head a wobble.
yes i lost the plot many years ago sean! 
i didnt want to cut a hole in my tank plus id have to upgrade electrics etc.also the water doesnt get hot enough to make a difference on add ons and first cleans from just using an immersion.i do a lot more than "a few first cleans a year"as you put it.same with add ons.ive picked up 2 full f/s/g cleans with conny roofs in the last week alone.
i look at it this way......say i earn 20k more than you a year sean.over the course of 10 years thats £200,000.take off £30,000 for a new van,hot diesel system,new poles etc and another £9,000 on diesel(if using heater every day) for running the heater,say another 3k on servicing,parts etc ive still got £158,000!
most vans ive had previously have been over 10 years old and lasted me roughly 3 or 4 years so my van will easily last 10 years and then itll only have around 40,000 on the clock so ill still get a few grand for it.
Confirmed, you have lost the plot, if you don't want to cut a hole in your tank and upgrade your electrics then pay somebody to do it, £300 to £500 should be more than enough.
The immersion guys will disagree on it not being good enough but that aside, as a sole trader with a full round how many first
cleans will you be doing yearly ? let say 12,
Same goes for fsg cleaning, are you intending to drop window cleaning over this type off work, I very much doubt it.
So lets look at the real savings, lets say hot saves you 15min on a first clean therefore 12 first cleans is 3 hours a year, and lets say it saves you another 12hours over the year on fsg cleans, that wouldn't even cover the cost of fuel never mind the original outlay.
Here's how you make window cleaning easier, clean as few properties as you can for as much money as you can charge, its not
constantly having to clean more to cover silly expenses, that rules the same no matter how much your earning.