There was a web site, sorry i cant find it now, where some old fella had cut a door into the rear corner (it was a splitty) of his bus walled it within the engine bay so it was isolated, was very well done again not a cheap option but it looked well trick....
Saw an early panel that had a factory door opposite the slider for a gas bottle again boxed off internally, think it was a utillities van from northern europe somewhere again a nice solution.....
You could buy a torodial tank as used on lpg conversions they fit in the spare wheel well, fit a wheel cover over and your laughing speak to one of the lpg conversion companies abot cost....