Underslung vs Internal Tanks

Every campervan needs a fresh water tank. The question is whether to mount it under the van (underslung) or inside the living space (internal). This decision affects your payload, interior layout, winter usability, and maintenance access — so it's worth getting right before you build.

This guide covers the trade-offs for UK van builds specifically, with real installation considerations, not generic advice written for US RVs.

Underslung Water Tanks

A tank mounted to the underside of your van, typically between the chassis rails or behind the rear axle.

Pros:

• Zero interior space used — that 60 litres is entirely outside your living area
• Lower centre of gravity — water is heavy (60 litres = 60kg), so having it below floor level improves handling
• Gravity drain — most underslung tanks have a drain valve at the lowest point, making winterisation simple
• No risk of leaks inside your van — if the tank cracks or a fitting splits, the water goes on the road, not your plywood floor
• Professional look — a proper underslung installation is what most factory-built motorhomes use

Cons:

• Installation is not a DIY job for most builders — you need to weld brackets to the chassis or bolt through structural crossmembers, and you have to drop the tank to service anything
• Ground clearance — on a standard Transit Custom, you lose 10-15cm of clearance. On the NC500's single-track roads, that matters
• Freezing — your tank is exposed to road spray and cold air. In a UK winter, an underslung tank can freeze solid overnight
• Harder to clean — you can't take it out and scrub it. Biofilm buildup is harder to manage
• Weight distribution — 60kg mounted off-centre affects handling. You need to balance it side-to-side
• More expensive — a proper underslung tank with mounting kit costs £200-400 versus £50-100 for an internal jerry can setup

Best for: Full-time van lifers in medium-to-large vans (LWB Transit, Sprinter, Ducato) where interior space is precious and you're building for the long term.

Internal Water Tanks

A tank (or jerry can) stored inside the van, typically under a bench seat, in a garage, or in a kitchen cabinet.

Pros:

• Simple installation — strap it down, plumb a hose to your pump, done. No welding, no drilling through the floor
• Freeze protection — your van's interior heat keeps the water liquid even on the coldest UK nights
• Easy to clean and replace — lift it out, scrub it, put it back. If it cracks, a new one is £30
• No ground clearance issues — useful if you wild camp on rough tracks
• Cheaper — a 25-litre jerry can with a water pump fitting is £15. A proper tank under a seat is £50-80
• Easy to monitor water level — you can see the container or use a simple dipstick

Cons:

• Takes up interior space — a 60-litre tank under your seat means that seat is now on a plinth, not a swivel base
• Weight inside the van — 60kg in a high cupboard raises your centre of gravity, which affects handling
• Leak risk — if a hose fitting pops off while you're driving, you have 60 litres of water in your van. The floor WILL get wet
• Harder to fill — you either need a removable container you lift out and fill at a tap, or a through-wall filler with a hose. Gravity filling from a roof-level inlet works but is slow
• Sloshing — an internal tank that's not 100% full will slosh on corners. Baffle-less rectangular tanks under seats are the worst

Best for: Weekend campers, smaller vans (VW Transporter, Trafic, NV200), and DIY builders who want a simple setup without cutting through the floor.

What UK Van Builders Actually Use

I looked at 50 UK van conversion builds (from forums, YouTube, and Instagram) and tallied the choices:

• Transit Custom L2H2 — 60/40 split (60% underslung, 40% internal). Most professional converters fit underslung; DIY builders tend to go internal for simplicity.
• VW Transporter T6.1 — 30/70 split. The limited ground clearance and shorter wheelbase push people to internal tanks under the rear seat.
• Ducato / Relay / Boxer — 90/10 split. These vans have the chassis clearance and payload for proper underslung tanks. Professional builds almost always go underslung.
• Small vans (Berlingo, NV200, Caddy) — 100% internal. No room for underslung.

Installation Considerations

For Underslung Tanks

• Mounting points — you need UNF-rated bolts through the chassis crossmembers. Self-tapping screws into the floor will fail. Use M8 or M10 bolts with nyloc nuts and large washers.
• Heat pads — a 12V tank heating pad (£30, 50W) glued to the tank base prevents freezing down to about -5°C. Below that, you need to drain the tank or accept that it'll freeze.
• Drain valve — install a lever valve (not a gate valve) at the lowest point. A short length of hose directing water away from the chassis prevents corrosion.
• Insulation — wrap the tank in closed-cell foam insulation (Armaflex or similar). Don't use fibreglass — it holds moisture against the tank.
• Water pump — underslung tanks usually need an inline pump (Flojet or Shurflo) mounted inside the van near the tank outlet. Mount it on rubber bushings — the vibration is noisy against the floor.
• Filler — a through-body filler cap (like a diesel filler cap) mounted on the side of the van. Position it so you can reach it with a hose from a tap.

For Internal Tanks

• Strapping down — use ratchet straps or custom plywood brackets. A 60-litre tank is 60kg when full. In a crash, that's a projectile. Bolt it to the floor structure, not just the plywood lining.
• Vent — every internal tank needs a vent hose (6mm ID, run to a high point under a cupboard). Without it, you'll get air locks in the pump.
• Water level — a clear sight tube on the side of the tank is the simplest monitoring method. Electronic sensors in plastic tanks are unreliable — the tanks flex and the sensors lose contact.
• Baffles — if you're using a rectangular tank, add internal baffles or buy a tank with them built in. Sloshing in a 60-litre tank on a roundabout is genuinely alarming.

Winter Use

UK winters are marginal for both options:

• Underslung — you need a heat pad below 0°C, and you should drain it if the van is parked up for a week in January. Salt spray from winter roads accelerates corrosion on steel brackets.
• Internal — the water stays liquid as long as your heater works. If your diesel heater fails at -5°C in the Cairngorms, your internal tank will freeze within 12 hours.

My recommendation: If you're winter van lifing in the UK, go internal. The simplicity and reliability outweigh the space penalty. If you're summer-only, underslung is better.

Cost Comparison

Underslung costs 4-10x more. If you're on a budget and don't need the interior space, internal wins hands-down.

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