I'll give you my views on LPG. I've had a Jeep Cherokee and a Grand Jeep Cherokee, both absolute monsters when it comes to weight, I've pulled well over a tonne on trailers using them on LPG and there coped very well. Running LPG on my 3.0 Jag I can tell you that LPG gives almost the same MPG just costs less, it's slightly less quick under acceleration but so little you'd hardly tell.
I'm still looking for a van and I'm paying great attention to the LPG one's out there, if you're going to be carrying a lot of weight then a 13-1400 small van wont be that good full stop.
You'll get more MPG from Diesel for sure, but then do the maths with the prices at the pump. I will be looking at converted vans rather than looking to convert one myself, the cost of conversion compared to your fuel savings is something you can work out if you have a good idea of your annual costs.
Diesel vans are always going to have more torque when it comes to pulling weight though. But having had experience of LPG in car's and Jeeps I don't see it as a problem at all.
Look to Europe and see how many drivers use LPG over there, loads! Don't ever crash into a Spanish cab driver as they normally carry a calor gas bottle in the boot

Whatever you chose it's certain that the TAX will increase with popularity though, Diesel was originally supposed to be cheaper until people went over to it, then the TAX man wanted his slice. Originally only farmers used it, and grew their own fuel during the war!!
I would still look into electric Paul, get one on test and fill the bugger up with all your gear, see how it copes. Then look into pricing a PV system for your home. I'll be they make you pay fuel duty on the electric for the van though. Utter numptys they are lol