Your question is tough to answer without any context: mileage, history, etc. In my opinion, the condition of the truck and camper is a more important topic than the engine. These things are not cheap (unless you are used to maintaining a private aircraft or yacht). E.g. if it is an 08 ER with leaf springs/coils and the 22.5's and you want to upgrade to Earthroamer's air suspension, beadlocks and military tires (make sure you check your rear axle weight vs GAWR vs tire load chart), ER will charge you about the same amount (or more) for that conversion versus what you will spend swapping the engine with a good diesel shop.
Any competent diesel shop can swap a 6.4 without lifting the cab. If I ever have a serious engine problem, I will probably replace with a DFC "Tow" 6.4 though there are other good reman 6.4 options. "Dirtwhiskey's" comment above is accurate as well.
If the truck and camper are in good shape, the price is fair to you AND you know you want an Earthroamer, I wouldn't hesitate because of the 6.4.
I've had a 2008 6.4 Earthroamer which I bought new and it has been great and all over North America with many more trips to come.
I've had a decent number of diesels over the years, european and American, old and new technology. The crux issue with any newer diesel is the combined effects of electronics and emissions systems, both of which result in newer diesels tending to be more rube goldberg contraption than simple and reliable powerplant. Evidently what we have today is the very best product that top tier engineers can come up with when buyers demand quiet, luxurious 400hp/1000ft lbs+ vehicles that'll tow 30k lbs+ all day matched with ever stricter government emissions rules.
The 6.4 Powerstroke needs regular, proactive/preventative maintenance and a realization that it is a great truck overall burdened with a poorly contrived emissions system. There are plenty of them driving on and offroad every day, all over North America. Most every US diesel since mid 2000's is a technological marvel when it comes to power and capabilities but is going to be a more complex and far less forgiving experience for the owner than the 5.9/7.3/OM606/etc. diesels of yesteryear
I'm dedicated to regular maintenance and drive my rig like the ~17k pound camper that it is: I take it easy and enjoy the trip. It does best around 60-65mph on highway and gets about 10-12mpg combined. With 90 gallons of fuel I have decent range and 90 gallons of freshwater and 3 large solar panels on the roof recharging two GPL-8DL Lifeline AGM batts, diesel cooktop, 2 espar airtronic+1 espar hydronic for both heat and hot water, it can be off grid for quite a while, even in extreme conditions. I realize this setup isn't the latest/greatest, but it works great and has been absolutely reliable, esp. in the cold where some campers might struggle. It is the XV/LT model (not the longer/heavier "stretch" XV/LTS). I have 43" tires on a custom leaf and coil suspension (no air susp). I've had zero engine problems.
