I get about 180miles from 56l of LPG. It's not the most fuel efficient XJ with the lift, tyres, extra weight etc. but it is using a multi-point LPG system. I think 400 miles tops from 110l is what you should work to.
The bigger the tanks under the boot floor the more clearance you'll lose. The diameter of my tank (56l fill) means a loss of 2-3" compared to stock petrol tank.
I have thought about the two LPG tanks and a small petrol tank elsewhere under the floor but I do so few miles now it's not worth pissing about with.
I can't help thinking these days it's got to be best to have a small car, probably diesel, that gives high mpg for all the boring trips we all make. Keep the Jeep for fun journeys and when you need it.