The space between knowledge and opinion is very narrow. You will get a lot of opinion here, but most of it comes from knowledge acquired from experience. Difference experiences lead to different knowledge and thus different opinions.
Engine philosophy between GM and Ford is very different. Ford doesn't like large displacement engines to be all wrapped up at high RPMs to deliver torque and horsepower. Although this might be changing with the new Godzilla V-8. If you dig around you'll find that Ford engines develop most of their torque at lower RPMs so you don't need to wrap the engine up and have it screaming to be performing. GM seems to take the opposite route with a torque curve which generally means that you get all of your torque and power well above 3000 RPM, which is a lot of revs for a large displacement engine. And the 8.1 reaches peak HP well over 4000. Plus, it is very thirsty, I think more so than the V-10.
The V-10, especially the later ones are very reliable. The blowing spark plug issue was something that happened on the early ones, and if you find yourself a 1997-2001 V-10 with some miles on it, it has either had the issue and got it fixed or had it corrected before a failure and is unlikely to give you any problems.
If you are going to buy a used motorhome you will have to do some work to rehab it a bit and make it your own. If you are fortunate you can find a seller which has kept it garaged, carported, or bagged during the winter and kept up on most of the basic maintenance activities. Delam happens when there is a water issue or leakage which isn't caught early and dealt with. I recommend not buying a unit which has extensive visible delam, so go over anything you are thinking about buying with a fine tooth comb.
The older the unit the more likely you can find delam, even delam which isn't so much water related, but age related. Delam is a reality as hardware gets older. If a unit has been garaged in climate controlled places, delam is rare, but finding a motorhome which has been garaged its whole life is really difficult.
Try to find a used unit in which the previous owner has great documentation of their maintenance and upgrades. This is generally a good sign of a unit which should support you well in the future. Also note, that with a travel trailer your maintenance has been limited to "house stuff". The moment you buy a motorhome you have "house stuff" and "chassis stuff", and some of it blends together. And also keep in mind that you are buying a heavy duty truck, whether Class C or A. So, when you have to do maintenance you will have quite a bit of added expense, and you will need to have a mechanic which can work on heavy rigs. Not all mechanics can lift 15,000-20,000 lbs., not even dealers. This is why a lot of folks do buy a truck and have a trailer, because the truck can be used year round and fairly easily maintained and eventually replaced. I don't know too many people that have truck/trailer combinations in which the truck isn't a daily driver.
Getting a Class A without slides after about 1997 is a little rare, and you have found the few that didn't offer slides. Tiffin's are great, and probably the best of the three brands you have shared. But, if you are avoiding the slide out because you are concerned about the maintenance or the condition of a nearly 20 year old slide, that is understandable, but also shouldn't be a deal breaker.
Again, just like delam, look over your used prospect, make sure to exercise the slide a number of times before you buy it, and you might want to get a second opinion from a reputable RV maintenance shop, or at least have them "tune it up" after your purchase, before you go on a longer trip. If the owner won't let you exercise the slide a number of times before you buy, that is a red flag. Take somebody along to watch and listen to the slide on the outside and get their feedback. Did it move smoothly in and out and not appear to be hanging up on anything? Did the motors sound "normal" or did they seem mechanically strained? Look at the roof of the slide out, what condition is it in? Is it dirty or is the membrane in need of maintenance?
To be honest, prior to slides, manufacturers typically made units longer to provide more living space, storage space and features. If you were to look in the mid to late 90s, you'll find a lot of Class As with no slides, but to make space they tend to be longer, 35-40 ft.
Storage and access to storage cubbies and basements is going to be different between class A and C. I like As better because their basement storage access is generally through full sized doors. I haven't met an A built in the last 20-25 years or more which doesn't have really great basement storage and access to it. As also tend to have wet bays or service bays for all of your hookups and sewer dump. These bays are all inboard and protected from road debris and generally insulated from extreme temperatures, and often have heating ductwork to help prevent freezing.
My guess is that your mini-van/tent trailer combination have worked out well for you because you had plenty of room in the van for all your gear. Be sure to visualize in some way all the gear you take with you in the van and determine if the motorhome you eventually buy will fit it all. Again, I think Class As will be better equipped to handle all of your gear over a Class C.
Class Cs vary in terms of how their hookups are handled, but most have road exposed sewer dumps, small access ports for fresh water and electrical. Convenience and consolidation mean a lot to me, so I really value a single service compartment over having a lot of small nooks and crannies to do various stuff or service hookups.
Whatever you move into, it is going to take some getting used to considering your old way of RVing and it certainly won't be the same as what you have been doing. Happy trails.
