Given the choice of buying 2 new truckmounts a LPG or a diesel I would go for the diesel.
I would think a Diesel engine running on red diesel would be cheaper to run than a LPG engine even though LPG is cheaper to buy. And Diesel engine are much more industrial, if you look at machines that are designed to run 24-7 they always have Diesel engines never LPG.
Diesel engines are not populer with truckmount builders as the initial cost of a Diesel engine is much higher than petrol and they are conciderably heavier than an equal Hp petrol engine .....so makes the whole truckmount a lot heavier, more expensive and it has a bigger footprint in the van.
They also burn cooler so if you are running a heat exchanger to heat your water a diesel won't give as hot water as a petrol engine of comparative horse power
Also a Diesel engine is much much simplier in design, get a big petrol and you get into engine management systems which are a minefield of problems, a Diesel engine is incredibly simple in comparison which is why they are much more reliable