Much of this is personal and much depends on background experience, so some questions may help.
How do you feel about gas versus diesel and how much travel versus just living in one spot?
My personal experience with diesel is limited to watching others while I have a fair amount of experience with fixing gas. That leaves me to look at how few miles I put on an RV and how much trouble it would take for me to "relearn" the field if I changed. I have a brother who is a fully trained, experienced, etc. mech and he is not having a fun time with the new truck!!
Big time mileage makes sense for the diesel but for lower mileage that most RV get, the gas engine will likely outlast the user and with less pain. That's certain to get flak from the diesel folks as they like what they like but that is my view!
From there, I might look at class A for more usable front end space but recommend something smallish like a 30 foot range as it is less limited in where you can use it. Bigger units are often not able to get into the spaces which we prefer like state and national parks, etc. which are often smaller spaces and roads.
I recommend Winnebago for a number of reasons such as they do have a definite plan for building each RV and also put those plans and info up online so that you have a much better chance of fixing things as well as being able to FIND those things!
IF you need to fix an inverter, It's a big help to know you have one and where it is hidden!
Being a former mech, can you get info on the diesel chassis better than gas or the other way? Winnebago does not do much info for the chassis so you have to dig that out from other places.
A great place to start on the info search is here:
One definite place to not skip is the parts catalog as it is not just parts but they also have the layout of each rV as the first page! That is one place you might look if seeing an RV advert and not knowing what exactly it has. Go to the parts catalog and it shows you and if you drill down far enough, you can look at the RV from every angle and look to see where the drain lines go! But for buying/shopping, it can cut hours of chase off searching for things like where the TV is located, etc.