If hot water was an option in a 84
Bigfoot, which I'm not sure of but I would guess it was, then it is likely possible to install one. They're basically all built the same, some just have extras. But the shell and cabinets and all that should be almost exactly the same, since they tend to use a "cookie cutter" approach to save time and money. No need to completely reinvent the interior for each different option package.
I have never had a
Bigfoot without hot water, but I'm guessing it will be sort of involved. You would need the hot water heater, new
propane line routing for the burner, then you'd need to run the hot water water lines and install a faucet with both knobs.
Hot water heaters tend to be the same. Probably my 91 heater would fit your 84. There are only so many manufacturers who make RV appliances, and they have to fit a wide range of different RVs, so things tend to be pretty interchangeable for similar sized trailers.
I think this would be more of something you'd do if you were gutting and rebuilding a trailer.
I had a camper with no hot water before. No big deal, for me. The only time I use the hot water in my current camper is for showers. I don't bother firing up the water heater for dishes. I just put a little cold water in a pot and heat it on the stove. So for me personally, without a shower, I would definitely not think that all the work of installing a heater would be worth it. But you'll have to decide that for yourself.