With extremely conservative hwy driving, my TB45E gets about 25litres/100 on ULP, maybe 10-15% better on premiumULP on long runs. (4 adults luggage, no roof rack, no trailer, 2" lift)
On LPG it's 33/100 - so you're on the right track not converting IMHO.
These numbers would be heavy due to dual fuel and resultant difficulty in finding "sweet spot" in the tuning for both fuels (from what I've heard).
Haven't taken it on sand, but is "confidence inspiring" on long gravel roads!
It's a bit of a verge ornament at the moment - only kept for minor outings as I don't use LPG if I can help it, leaving me with 30litre sub tank of ULP...
Suggest u stick to standard profile tyres. Bought mine with 285/75/16s. Struggled keeping speed up long inclines. Seems better with 265/75/16s, but I haven't driven it that much since the changeover.
I prefer driving the TB45E over the TD42T (both are stock - no mods), as long as I suppress any thoughts about fuel economy!