One tank is more than enough. Our 845 has two 20# tanks but with several weeks in winter and summer trips we have never used more than 10# on a trip. Batteries are a different matter completely. With two batteries you can go twice as long before needing to recharge with the truck's alternator by driving or plugging in at a campground with 110v AC. Adding a second battery to a camper not built to hold two batteries is seldom easy to do and a path not worth taking. Get a camper with two batteries (and venting for them) straight from the factory if you want to spend time off the grid.
I built a rack to hold a second battery under my 845 camper but would never buy another camper that did not come from the factory with a place to store two batteries. The Lance 830/850/855/855S campers have an option for two batteries, but not the 865. The 865 is a lighter camper more in the weight range of the Wolf Creek campers.
Don't worry about the width of the camper so long as it is the length of the camper and truck overall and the required truck's length and width that impact where you can go the most. We go most of the time to small USFS campsites where backing in is easy with our extended cab short bed SRW truck with the 845 camper (about the same size and weight as the 855).
Weight is important as the more the camper weighs the less extra stuff you can take and this includes the tongue weight of a trailer that is being towed and fluids in the holding tanks while traveling. Don't trust the published dry weights from the manufacturers as they want to sell their campers as being as light as possible. Our 845 has a sticker on the back stating the weight as it left the factory as being under 2700 lbs. but with the battery added, storm window, roof rack, AC, microwave, and empty holding tanks it weighed when measured at a nearby CAT scale as being 3200 lbs. and that was before be put in food, gear, fresh water, or any other necessities. Fully loaded for a trip the camper weight is nearly 4,000 lbs. on the rear axle.
Unless you go with a truly lightweight camper like a Northstar popup the truck needs to be able to handle a 3,000 lb or greater load that also has a high center of gravity. My stock 2011 GM 2500HD was able to handle the 3200 lb camper with no modifications. But to improve handling and be able to add gear and food and water I did make two modifications. I added SuperSprings at the rear axle to add 1400 lbs. to the stock springs load capacity, and I replaced the factory supplied tires rated at 3195@80 PSI with Nitto tires rated at 3750@80 PSI which increased the rear tires' load capacity by 1110 lbs.
My shortbed truck is 19" shorter bumper to bumper and has a 14" shorter wheel base than the longbed version and this definitely makes it easier to go on western USA forest service roads built by the government to subsidize private logging operators with their Caterpillars, also makes it much easier to get into tight USFS and NPS campsites.
The 845camper is not as wide as my fully extended camper mirrors for way of reference. At 93" it extends past the center line only 1.5" less than the current 855 camper and only 3.5" more than the current 865 model. Realize that half the total differene in width is going to affect the actual overhang of the side of the camper so the impact is less than one might expect. Take the center of your truck's bed and from there measure out 48" and you will have the outer edge of a 830 or 855 camper in the truck. The extra 5 inches is not going to make a difference in where you can go but the extra 10 inches does make a significant difference inside the camper where space is at a premium (and why slideout sections are popular).
A separate but related item to add to whatever camper you get is a Trimetric charge monitor. It will tell you exactly how much charge is in your battery and how much is going out and how much is coming back in at any point in time. The Lance supplied LED electrical gauge is not at all accurate in terms of the actual state of charge and provides no information as to how much charge is going in or coming out of the battery. For less than $200 it is a very worthwhile addition and something that Lance should provide or at the very least provide as an option.