If you are happy using a car and a trailer system, why change?. Just because not many on here use a trailer (that post about it anyway), doesn't mean there is anything wrong with it.
If you really want a change though, and don't want a trailer, but need 4/5 seats, there are options. A crew cab van would be my preferred choice, but there are a few on here who use a double cab pick up.
If you are a solo operator, you can probably get away with a 350l tank, but certainly no more than 500l (that is what i use). If you work as a 2 man team, then you would need at least 650l, which would obviously affect your choice of vehicle.
Whatever vehicle you get be very careful with payloads as experience has shown that the 'quoted payload' is not in any way an accurate representation of what size tank you can legally put in there!. If you read all posts on here, you will have an idea why. Me, for example, i have a transit connect with a 900kg payload, i have a 500l tank, yet on a customers weighbridge with a full tank, i was 40kg under my gvw. Another recent post had someone with a 650l tank in a vivaro going on a weighbridge to find that not only was he well over weight for his 2700 (gvw in kg's) version, he would still have been overweight with the bigger, 2900 version!. Someone else, had a 350l tank in a combo, 787kg (or there abouts) payload, but on a weighbridge he was only about 90kg under gvw. Now i have been labelled a 'keyboard warrior' for expressing my 'opinion' on these matters, but i am not too bothered about that. I just want to make sure you buy the right van for your needs, as even though it may be statistically unlikely that you get stopped by vosa with a full tank of water, it could happen, and has, to at least one guy on here!.