I've got to agree with Slowmover on most of his points.
An 18' trailer is going to weigh in at 5-8000 lbs. Axles are rated around 4-5000 in the smaller sizes.
having towed single,tandem and tri axles, lets just say I love my tri axle in cross winds
The 41' beast you see below tows better than my old tandem axle 35'.
The propane lines can be run below the floor but inside the belly pan. Yes mine came factory with an insulated full undertray. Its great in the winter. The tanks dont have to be fully outside but have to be in a sealed compartment with an open bottom.
I suggest the fresh tank go in a cabinet inside, under the bathroom sink is common. Balance the black and grey tanks around the axles. The grey will fill up at least twice as fast, trust me.
Storage space is a premium in RVs. Dinetes offer a huge advantage over table and chairs.
All that said, check your vans tow weight and especially the tongue weight.