As others have said your use case is the most important factor here.
1. Do you always use campsites with EHU
2. Do you already have Gas on board
3. Do you want hot water
4. How often do stay in the van when its cold
5. Does noise disturb you when sleeping.
6. what space do you have as you are retrofitting in an already built van.
If you are limited on space internally due to the fit out, under the floor or on the roof are your best options, I say roof as you can aircon units with a heat pump, but I've never looked at these because of the power consumption. Under floor the choices are not bad as many have mentioned there are the Eberspacher and Propex options (Propex do an under vehicle model) if you use sites with a hookup consider getting a version which is dual fuel i.e. Gas / Electric, that way you don't burn fuel when you could be using.
Gas burners are not as noisy as diesels, however if you are going for blown air the blown air itself can be pretty noisy, which is fine during the day but can be disturbing at night when it switches on and off. Constant noise is easier to ignore. We had a propex in our old van and couldnt use it at night because of the noise in the cabin, so just turned it on in the morning before getting up, you can wrap up warm when your in bed so not that bad really.
If you don't already have gas on board and you go for a gas option you need space for the tank, as your van is already fitted under the floor is probably the best option, there are some easy to fit kits available for most commonly used vans, to fill these up you need to find a garage with LPG which if you travel widely (i.e. outside of the UK make sure you have adaptors)
If you only need a heater occasionally and always have a hookup, take a small electric radiator, this will easily heat your van, silently, quickly and cheaply.
Some of the diesel underfloor options include a water heater, this can be combined with a heat exchanger on the engine which will provide heat whilst driving again saving you burning fuel unnecessarily, and you get hot water without wait at your destination.
As an aside an alternative for hot water that we had on our old van for hot water was a marine calorifier which heated up the water off the engine when driving and also had an electric immersion for when on sites with EHU, as the tank was insulated water would stay hot for about 12 hours after driving, enough for washing up, strip washing.. again totally silent.
I am assuming that you don't have space for a Truma or Alde type system though this depends if you have a large storage area under your rear seating for example with an outside edge for the vent/air intake, you could have a truma type system, but think about where the internal vents need to go and whether there is space for the pipework.
You can rule out Alde on a fitted vehicle as it requires a lot of installation.