Hi,
You have a really interesting 4X4 van to start with. Your plan to add a high top roof is doable, and will expand the room inside. I know someone towing a 34' Airstream with a then new 1979 E-350 that was not a extended van(for shorter rear axle to tow hitch length) and a then powerful 460" engine in it. This was before the 6.9L was even considered in the vans. And the 460" was much higher power than the non-turbo 6.9L of that day.
First they installed a pop up roof, then replaced that with a fixed 24" tall fiberglass roof. Yes they can install them at any time you like. It might be expensive though.
First of all, I would consider installing a Napa 4070 water coolant filter on the engine. It goes into the heater hose line, with a kit #FIL 4019. I put mine behind the bumper, and can put a bucket under it when I change the filter each year. This will keep minerals out of the radiator, oil cooler, and EGR cooler. Keeping them clean will prolong the engine life.
I installed a digital temperature gauge on my transmission. I installed the sensor on the pressure tap plug on the side of the transmission, and it gives a good indication of how hot it is running. The 6L diesel has plenty of power, and could overheat and thrash a transmission if you do not slow down on the mountain grades when the transmission is overheating. .
In my GMC 3500 class C, the transmission ran at around 235F on level ground on a 95F day. On the grades it was over 275F, so I had to slow down. I installed a huge transmission cooler once at home, and that solved that problem.
Back to the van. Some sort of cargo compartment is usually built, so that the bed can be made on top, and stuff stored below. By making the corners out of a wood like Poplar, or oak, it is very strong, and yet small and light. You could use 1X2" and it is strong enough. Cover with 1/2" plywood, and you can put something like sleeping bags or your bean bags on top of that to sleep.
You should install LED lights. You can search for LED light and find several posts on that subject.
You should install a extra "house" battery and a relay that is only energized while the engine is on, or perhaps the "Radio circuit" is on. It can be a 30 amp fog lamp relay, and some #10 wire from the engine battery to the second battery, with the relay and some 40 amp automatic reset circuit breakers in between.
For a battery. Most Ford class C's use the drivers side battery comparment and can use a regular lead acid battery. Yet your diesel has two starting batteries, so that compartment is already full. You should consider a AGM battery that will not give off any gas vapors, and thus can be mounted inside the RV safely. A lead acid battery might require over $100 to make it safe, with proper venting outside.
You might want to consider a solar panel on the roof. They are really cheap these days, and by installing a 200 watt solar panel, you will not need to 'store' over 100 amp hours of power, because you can generate 35 - 50 amp hours daily, thus not need to store nearly as much for use over 3 days.
SunElec.comGood luck!
Fred.
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