I think 25 feet is going to be a bit too big for your Jeep Grand Cherokee. While the Jeep has good torque and should do a good job with a smaller van, it's small payload and rear axle capacity are likely to be the limiting factors with a van that size, and all the goodies you will want to carry in the back of the Jeep.

Most of the large manufacturers are not interested in custom builds. They work on a production line basis and try to standardise everything as much as possible. We found that when we were looking for an accessible van as my wife is confined to a wheelchair and we wanted something that would suit. The big boys weren't the least bit interested in talking to us so we flew to Melbourne to have a look around, that being the caravan manufacturing capital in Australia. We found about half a dozen small manufacturers who specialised in custom builds and finally settle on Evolution Caravans. Their factory is in Epping and the Sales office is on the Mornington Peninsular. In general we have been very happy with the Evolution. It has travelled all over the continent crossing it back and forth 6 times now over a 8.5 year period. Done about 90,000ks in it. I have just started needing to do some minor repairs to it now but it has been a sturdy van and would see them again for another.