LPG is certainly worth while if you are going to eastern Europe. It's widely available and very cheap. Can't see the gas bottle idea working though. there are problems with the connections, they are extremely heavy and I would have serious safety concerns about them. Just fit an LPG tank extra to the main petrol tank. That's the standard way to do it.

I had that on an V6 Petrol L200 Mitsubishi, a 60L tank netween the chassis rails, was well tucked up so no risk of damage. Best MPG I got was 30mpg towing a motorbike on a trailer. With LPG at that time half the cost of diesel that represented the equivalent cost of doing 60mpg, not bad from a 3litre engine!!

When I had a landrover 101 ambulance made into a camper I had a 100 litre tank under the rear bed. Fuel consumption on petrol was about 10mpg, on LPG 8, so the conversion paid for itself pretty quickly.

If you go that route there are a lot of cheap big engine petrol 4x4s out there because people shy away from them due to the fuel costs. A good quality conversion (and it pays to have a good quality one) is £1400-2000

The other big incentive for older vehicles in the UK is road tax. A big engine vehicle (I now a Dacia isn't one of those!) can be eyewateringly expensive to tax, wheras a pre 2000 model is more reasonable.