You might want to peruse the used Sportsmobiles listed on their website. They generally have internal tanks and relatively good ground clearance.
Another option might be a newer Promaster or Ford Transit van with a simple conversion by VanWorks in Colorado, which includes simple internal systems. VanWorks can provide a basic conversion for $14-18k and you could probably find a closeout Promaster or Transit on dealer lots for the low 30s.
Although the Transit and Promaster vans don't have as much ground clearance as the old E250 they're probably better than a PW or LTV with low hanging tanks and running boards.
