Yup, multivitamins are good. Try it as prescribed on the bottle for 3 months, then stop, and see after two weeks of no vitamins if you feel differently. Just make sure to get a good brand of multivitamin. Sisu is owned by a pharmaceutical company (note: company that profits from sick people needing emergency medicine) as well as many other vitamin companies. Health food stores and vitamin shops often have knowledgeable staff who can tell you what's worked for them and their customers.
I also really like getting something simple like alfalfa powder from the bulk section of a health food store (as long as they store in properly and it's not horribly stale) and either putting it into capsules (also available at most health food stores) or putting it in my food (strong 'green' flavour). (Or if you have access to a dehydrator and grinder for making your own green powders, which would be best)
Also I usually travel with dried wild animal blood (usually deer) for putting into soups, but that's kind of hard to obtain. I got mine from a roadkill deer's bloodclot. I laid it out on some fabric on a dehydrator and waited till it turned hard, and then put it into a coffee grinder till powdered, put it in a bag and voila, super nutritious formula. Maybe you can make friends with a hunter? Or find some really fresh roadkill, but I believe if blood is old and unpreserved it can be harmful to your health.
My health has been affected enourmously by carrying a stove and a pot. Any dumpstered veggies all go in there, a good way to get variety. And with a teaspoon of deer blood, lots of extra vitamin and mineral boosts!
Garlic is really good too and keeps quite well. I also carry a can of tuna in my pack, and a can opener on my multitool. I've also sprouted mung beans in a variety of situations, in tupperware containers.
Mostly before a trip I try to do a big dumpster run and dehydrate everything (can be done in an oven on low heat, in a makeshift tipi, on an old window screen supported by some pallates in the sun, etc.). I usually end up with some veggies and some meats that are dry, light, keep well and are compact.